Tbis is an action in equity brought by a taxpayer of Milwaukee county to enjoin tbe issuance and sale of" negotiable county bonds amounting to $450,000 for tbe purpose of building a viaduct across tbe Menominee valley, connecting Grand avenue on tbe east side of said valley witb Grand avenue extended on tbe west side thereof. Certain-corporations and firms which bad contracted to purchase said bonds were made defendants, together witb tbe county officers. Tbe bonds were about to be issued under tbe provisions of cb. 444, Laws of 1903. Tbe defendants answered jointly, and,. tbe plaintiff having demurred generally to tbe answer, tbe demurrer was. overruled as a demurrer to tbe answer, but sustained as a demurrer to tbe complaint, and tbe plaintiff appeals.
Tbe first and most important contention made by tbe appellant is that said cb. 444 is a special or private law, and heneé-is unconstitutional under tbe provisions of sec. 31, art. IV,. Const., which prohibits tbe enactment of special or private laws for various purposes, including tbe laying out of highways, assessment or collection of taxes, and for tbe granting of corporate powers or privileges. Tbe law thus attacked is entitled:
“An act to authorize tbe building and construction of viaducts across valleys, gullies, running streams or railroad-tracks by counties in tbis state of a population of 150,000 or more and for tbe issuing of county bonds therefor.”
Tbe first section provides that:
“Tbe county board of supervisors of any county within tbis state which now has or may hereafter have according to any-state or national census taken a population of 150,000 or more, is hereby authorized and empowered to erect, construct and maintain any viaduct or bridge over and across any gully,, river or valley, or railroad track or tracks agreeable to thé conditions and provisions of tbis act and subsisting laws applicable thereto, when in tbe opinion of such county board tbe erection of such viaduct or bridge shall be for tbe best inter*347ests of tbe county and inhabitants thereof, which opinion, shall be rendered by resolution duly adopted by the county board of supervisors of such county, at any legal meeting, thereof. Such viaduct or bridge shall be constructed of such length, width and height as the said county board of supervisors may by resolution determine.”
The remaining sections of the act contain detailed provisions governing the manner in which the work shall be done, the contracts let, and the bonds issued, as well as provisions for the levying of taxes to pay the principal and interest of the bonds.
The county of Milwaukee is the only county in the state having a population of 150,000, and the claim is that this law is evidently a law passed solely for Milwaukee county, and is consequently a special or private law and not a general one. The ultimate and controlling question is whether classification of counties by population is a proper classification as relating to legislation concerning the building of public improvements. If it be proper classification, then the law is a general' one, notwithstanding the fact that there is now but one member of the class. If not, then the law must be held special. Adams v. Beloit, 105 Wis. 363, 81 N. W. 869. The general rules governing classification have been frequently stated,, and may be said to be briefly as follows: The classification must be based on substantial and real differences in the classes,, which are germane to the purpose of the law and reasonably suggest the propriety of substantially different legislation, the legislation must apply to each member of the class, and the classification must not be based on existing circumstances-only, but must be so framed as to include in the class additional members as fast as they acquire the characteristics of the class. State ex rel. Risch v. Trustees, 121 Wis. 44, 98 N. W. 954, and’cases cited on page 53 (98 N. W. 957). Applying these general rules to the law here attacked, it is at once seen that the classification is not based on existing facts. *348•only, but is so framed as to admit additional members, so that there is but one question, and that is whether the classification is based upon a substantial and real difference which is germane to the purpose of the law, and suggests the propriety of such a law to govern a county of 150,000 inhabitants while denying it to counties with lesser population. A marked ■and substantial difference in population is, of course, a real •difference, but does it suggest the propriety of greater powers in the matter of the construction of such public improvements than the ordinary powers possessed by smaller counties ? If this question related to cities instead of counties, then there would seem to be no doubt that it should be answered in the affirmative. That large and populous cities need better, safer, and more expensive highways and bridges for the accommodation of their teeming populations than small communities no •one will deny. The simple rural highway and the inexpensive bridge may be ample for all purposes in the village or small ■city, while for the congested conditions of the. great city the asphalt pavement and the broad and expensive bascule bridge may be not only proper, but absolutely essential to human safety. Classification of cities by population, and the enactment of general laws for each class according to its needs, has been so often approved by this court that it has become part of the fundamental law of the state. State ex rel. Risch v. Trustees, swpra. No good reason is perceived why the same •considerations, in a somewhat modified degree perhaps, do not justify classification of counties by population as well as cities. Indeed, this court has affirmed the validity of an act providing for the payment of a salary to registers of deeds in lieu of fees in all counties containing a population of 150,000 •or upwards, on the ground that the classification was proper, and hence that the act was a general act and not private or local. Verges v. Milwaukee Co. 116 Wis. 191, 93 N. W. 44. While the subject of classification was not discussed at length in this case, it is manifest that it could not have been *349decided as it was except on tbe basis tbat tbe classification of' counties by population in legislation relating to tbe payment of a salary in lieu of fees to tbe register of deeds was proper because germane to tbe subject; tbe idea doubtless being tbat in a populous county where many real-estate transfers are-made, tbe fees provided by law would furnish an extravagant' compensation to the register* while in small counties where-tbe transfers axe few in number tbe fee system would afford simply a' reasonable compensation. So this court sustained tbe validity of a law granting to town boards of supervisors-all tbe powers of village boards (with certain exceptions)' where tbe town contained an unincorporated village containing not less than 1,000 inhabitants, and held it to be a genera! act. Land, L. & L. Co. v. Brown, 73 Wis. 294, 40 N. W. 482. This decision could evidently only have been reached on the-basis tbat classification of country towns by population was-proper. Indeed, tbe language of tbe opinion on page 302 (40 N. W. 485) shows this to have been tbe basis, although tbe word “classification” is not used.
But, apart from these decisions, it seems apparent tbat a-county of more than 150,000 population is quite certain to-contain densely populated areas requiring means of intercommunication by way of highways and bridges far more elaborate and expensive than those which will fully satisfy tbe modest needs of a sparsely settled county, devoted principally to farming and containing only a few villages of small population. So we reach, without serious difficulty, the' conclusion tbat tbe classification of counties by population in tbe law before us is germane to tbe subject covered by tbe law, and tbat tbe characteristics of tbe class' suggest tbe propriety of such a law as applicable to tbe class, while refusing it to other counties not in tbe class.
A number of minor objections are made which will be-briefly noticed. Our constitution (sec. 3, art. XI) provides tbat any municipality incurring an indebtedness “shall, be*350■fore or at tbe time of doing so, provide for tbe collection of a direct annual tax sufficient to pay tbe interest on such debt as it falls due, and also to pay and discharge tbe principal thereof within twenty years' from tbe time of contracting tbe same.” Sec. 10 of tbe act in question also provides as follows:
“It shall be tbe duty of any county board of supervisors which shall have determined upon tbe building of any viaduct under tbe provisions of this act, at or before issuing any bonds ■in pursuance hereof, to provide for tbe payment of tbe interest and tbe ultimate payment of tbe principal, of any and all bonds which shall be issued under and by virtue of this ■act; and for that purpose such county board of supervisors is hereby authorized and required at or before tbe issue of any ■such bonds shall have been determined upon, to provide for the assessment, levy and collection of a direct annual tax upon all tbe taxable property of such county in which such viaduct shall be located, sufficient to pay tbe interest on such bonds so issued, and also to provide for the levy and assessment of a direct tax, sufficient to pay and discharge tbe principal of said bonds, as the same shall mature and to collect the same in the manner as other taxes are levied and collected by law.”
In attempted compliance with these constitutional and statutory requirements, the county board of Milwaukee county, at •the same time that it passed the resolution directing the issuance of the bonds, also passed a resolution providing that to meet the principal and interest thereof “there is hereby levied ■on all the taxable property of the county in each of the twenty years 1905 to 1924, inclusive, a tax in the sum of $22,500 for the payment of the principal of said bonds,” also in each of said years the amount set opposite each year, respectively, for the payment of interest (the amount of the interest levy for ■each year being specified). It is said that the board had no authority to levy a tax for any year beyond the current year, and hence that the attempted levy for future years is invalid, and the constitutional requirement has not been met. This contention logically means that no municipal debt can be incurred without'levying a tax to pay the entire principal and *351interest during tbe current year, a result wbicb is as absurd as it is plainly contrary to tbe plain import of tbe constitutional provision. So far as ordinary tax levies are concerned, tbe county board cannot make tax levies for future years, but it was just tbis difficulty wbicb tbe constitutional provision named was framed to meet and correct, as fully explained in tbe case of Kyes v. St. Croix Co. 108 Wis. 136, 83 N. W.
In tbat case it was beld tbat neither tbe constitutional provision aforesaid nor a statutory provision identical in terms with sec. 10 was satisfied by tbe mere passage of a resolution providing tbat there “shall be annually levied by tbe county board” sufficient sums to meet tbe principal and interest of tbe bonds as they fall due, for tbe reason tbat such a resolution was not an exercise of tbe county board’s legislative power to levy a tax, but a mere promise, or command to future boards to do so. We have no disposition to review or change tbe rule announced in tbat case. Following tbat rule, it is evident tbat tbe action taken in tbe present case was correct, and indeed tbe only action wbicb would satisfy tbe constitutional and statutory requirements. Tbe objection tbat some of tbe taxes might become delinquent and not be paid until after sale, and hence tbat tbe entire amount might not be in hand at tbe time when tbe annual instalments fall due, is not deemed of sufficient dignity to require treatment. Tbe levies provided for, if collected when due in regular course, will meet all instalments of principal and interest as they become due. Tbis must be beld to be a sufficient provision.
Tbe law in question requires tbe county treasurer to execute an additional bond with sureties, in double tbe amount of tbe proposed bond issue, within thirty days after it has been determined to issue such bonds, and before be shall receive tbe bonds or any money derived from tbe sale thereof. It appears tbat tbe county treasurer filed a bond July 1, 1905, wbicb was approved by the county board August 8, 1905, but was subsequently discovered not to contain all the conditions *352required, by tbe statute, and iu September following, and before receiving any bonds or money, filed a new bond containing all tbe necessary conditions, wbicb was duly approved by tbe county board. It is claimed tbat tbe proposed bond issue must be enjoined because of failure to file tbe bond witbin tbe required time. Tbis contention must fail. Tbe purpose of tbe bond is simply to indemnify tbe county against loss resulting from tbe failure of tbe treasurer to perform bis duty. Tbe .requirement as to tbe exact time when it shall be filed is plainly directory only, at least so long as it is filed before tbe treasurer’s duties begin. Neither tbe treasurer nor bis sureties could be beard to say tbat it was void because not given witbin tbe prescribed thirty days. Murfree, Official Bonds, §§ 321, 5S3.
Tbe law provides tbat tbe bonds issued under tbis act shall be “in tbe sum of one thousand dollars, five hundred dollars, and one hundred dollars each, tbe number of bonds of such denomination to be fixed by tbe county board of supervisors.” Tbe resolution of tbe county board provided for tbe issuance of $1,000 and $500 bonds, but not for tbe issuance of any $100 bonds, and it is claimed tbat tbe bond issue is illegal and should be restrained, because tbe law is mandatory and requires tbat all denominations shall be issued. Tbe law is doubtless mandatory to tbe extent tbat no bond can be lawfully issued under it unless such bond be of one of tbe denominations prescribed; but should its mandatory character be construed as extending any further ? We think not. If it bad been intended to require tbat all three denominations should be issued, with tbe idea perhaps tbat small investors should have an opportunity to subscribe as well as large ones, it would seem tbat tbe legislature would certainly have guarded the provisions by fixing at least a minimum proportional amount of tbe smaller bonds wbicb must be issued. Instead of any such provisions, however, tbe legislature expressly authorized tbe county board to fix tbe “number of *353bonds of snob denominations”- to be issued, tbns making it possible for tbe board to issue all three denominations and yet issue but one bond of $100. Thus tbe letter of tbe law would be fully complied with, but its spirit (on the basis that it is mandatory) would be set at naught. Again, tbe provision last quoted says that tbe county board shall fix the-number of bonds of “such denomination,” not of “each” denomination. Tbe word “such” must have been used advisedly, and must, it seems, refer to such denomination as tbe board shall determine to issue, otherwise it would seem to have no appropriate place in tbe sentence. These considerations lead' us to tbe conclusion that tbe law does not require that all tbe denominations shall be issued in a given bond issue.
Some minor objections are made which do not seem well •taken or worthy of special treatment.
By the Oourk. — Order affirmed.