246 



SCIENCE. 



[y. S. Vol,. XXIII. Xo. 581. 



of adaptive characters would proceed with 

 corresponding- rapidity. 



Illustrations of functional and geo- 

 graphic variation in the same animal are 

 afforded by the kangaroo rats (genus 

 Di2wdomys)—a, group of American mam- 

 mals highly specialized for life on arid 

 deserts. These ■ animals have numerous 

 enemies and a multitude of competitors, 

 which means that the struggle for existence 

 is always severe. They are of small size 

 and have big heads, big eyes, small fore 

 leg-s and feet, exceedingly long hind legs 

 and feet, and very long tails. The long 

 hind legs and tail are special adaptations 

 for saltatory progression— for leaping in- 

 stead of running. Another general func- 

 tional character is the extraordinary devel- 

 opment of the internal organ of hearing, 

 which forms more than half of the bulk of 

 the skull, enabling the animal to detect the 

 approach of its mortal enemy, the soft- 

 footed desert fox. 



In addition to functional variations of 

 this kind, which are general or common to 

 the group, there are others that are local in 

 character and confined to particular species 

 or subspecies. Of these may he mentioned 

 the increase in size of the hind foot in 

 forms inhabiting soft or yielding soils— an 

 adaptation that is even more marked in 

 certain other groups. 



Turning now from functional to geo- 

 graphic variations, we find in certain spe- 

 cies a marked decrease in size from the 

 north southward. In Dipodomys specta- 

 hilis, which ranges from north-central New 

 Mexico to southern Chihuahua— a distance 

 of about 700 miles— the actual decrease in 

 length is 52 millimeters (2 inches), or 14 

 per cent, of the total length. This is in ac- 

 cord with the general law of decrease in 

 size from the north southward, announced 

 by J. A. Allen many years ago. 



The kangaroo rats furnish illustrations 

 of still other matters of interest to the 



student of evolution. Passing the subject 

 of color adaptations, of which much might 

 be said, let us look for a moment at certain 

 facts brought to light bj^ a study of bodily 

 proportions. Tabulation of the measure- 

 ments of upwards of 500 adult specimens 

 of the several species shows that individual 

 variation is great, amounting in the hind 

 foot to 15 per cent., in the total length to 

 20 per cent., in the length of tail to 24 per 

 cent.— thus affording, ample material for 

 the evolution of new forms characterized 

 by differences of proportion— but none such 

 are developed. Indeed, the mean measure- 

 ments throughout the genus are remarkably 

 constant, the ratio of hind foot to total 

 length varying only 2 per cent. ; of tail only 

 4 per cent. We have here a case of wide 

 range of individual variation coupled with 

 sui-prising constancy of proportions. What 

 does this mean? It means, if I interpret 

 the facts aright, that all the species of the 

 genus Dipodomys have come to a halt along 

 a common line, like soldiers in a well-drilled 

 regiment, indicating that in the course of 

 their evolution from a generalized to a 

 specialized type they have already reached, 

 with respect to the environment and mode 

 of life, a state of equilibrium or equipoise, 

 from which any marked departure is in- 

 jurious if not fatal. The possibilities in 

 the way of divergence are shown by the 

 large range of individual variation, which, 

 however great, ceased long ago to operate 

 in the production of new forms. All de- 

 partures from the type are clearly disad- 

 vantageous and hence are promptly elim- 

 inated by natural selection. The operation 

 of dynamic causes has resulted in the pro- 

 duction of fixed conditions so far as the 

 proportions are concerned— and no further 

 modification need be looked for unless a 

 marked change should occur in the environ- 

 ment. 



SUBSPECIES. 



It is obvious from de Vries's. writings 



