logical inference and justified by its results. It is not claimed for the hypothesis of selective mating that it has a higher order of validity. _Use and Disuse._ As we have already seen, biologists are divided into two schools, one of which maintains that the effects of use and disuse[DE] have been a potent factor in organic evolution; the other, that the effects of use and disuse are restricted to the individual. My own opinion is that we have not a sufficient body of carefully sifted evidence to enable us to dogmatize on the subject, one way or the other. But, the position of strict equilibrium being an exceedingly difficult and some would have us believe an undesirable attitude of mind, I may add that I lean to the view that use and disuse, if persistent and long-continued, take effect, not only on the individual, but also on the species. It is scarcely necessary to give examples of the kind of change which, according to the Lamarckian school, are wrought by use and disuse. Any organ persistently used will have a tendency, on this view, to become in successive generations more and more adapted to its functional work. To give but one example. It is well known that certain hoofed creatures are divisible into two groups--first, those which, like the horse, have in each limb one large and strong digit armed with a solid hoof; and, secondly, those which, like the ox, have in each limb two large digits, so that the hoof is cloven or split. It is also well known that the ancestral forms from which both horse-group and ox-group are derived were possessed of five digits to each limb. Professor Cope regards the differentiation of these two groups as the result of the different modes of use necessitated by different modes of life. "The mechanical effect," he says, "of walking in the mud is to spread the toes equally on opposite sides of the middle line. This would encourage the equal development of the digits on each side of the middle line, as in the cloven-footed types. In progression on hard ground the longest toe (the third) will receive the greatest amount of shock from contact with the earth."[DF] Hence the solid-hoofed types. Here, then, the middle digit in the horse-group, or two digits in the ox-group, having the main burden to bear, increase through persistent use, while the other digits dwindle through disuse.[DG] On the other hand, one who holds the opposite view will say--I do not believe that use and disuse have had anything whatever to do with the matter. Fortuitous variations in these digits have taken place. The conditions have determined which variations should be preserved. In the horse, variations in the direction of increase of functional value of the mid digit, and variations in the simultaneous decrease of the functional value of the lateral digits, have been of advantage, and have therefore survived the eliminating process of natural selection.