VARIATIONS IN ANIMALS AND PLANTS 497 



organisms, notably in man, in other mammals and in birds. They 

 form a large factor in the development of the individual. The educa- 

 tion and training of the individual man produces functional variations, 

 as distinguished from innate peculiarities. But the groups character- 

 ized by functional variations are of the nature of ' ontogenetic species.' 

 There is no evidence that the current of heredity is affected by changes 

 of this kind, or that they have any direct effect in the formation of 

 true species. At the most, use or disuse of organs seems to affect 

 species only in an indirect way, as by the preservation of those most dis- 

 posed to functional activity, and which by such activity have been able 

 to meet better the demands of the environment. 



Collective Variations 



The preservation of individual variations with their extension to 

 posterity gives rise to racial changes, and these to the larger variations 

 which mark change in species. 



Collective variations are chiefly of geographical or geological origin: 

 Changes with space produce geographical species and subspecies. To 

 this category belongs the vast majority of species and subspecies recog- 

 nized in systematic zoology and botany. It is illustrated by the species 

 of wood warbler (Dendroica) mentioned in a previous paper. 



Changes with time produce geological mutations. It is a fact un- 

 questionable that a species will change on its own grounds little by 

 little with the lapse of time and the slow alteration of conditions of 

 selection. Nations change, languages change, customs change, noth- 

 ing is secure against the tooth of time. This is in general true, be- 

 cause with time alteration of environment takes place, events happen, 

 there is an alteration of the stress of life and with this alteration all 

 life mav be modified. 



That time-mutations in all forms of life do take place is beyond 

 question, and some have regarded these slow changes as the chief agency 

 in the formation of species. But the current of life does not flow in 

 straight lines nor in an even current. Species are torn apart by ob- 

 stacles, as streams are divided by rocks, and the rapidity of their forma- 

 tion is proportioned to the size of the obstacle and the alternations it 

 produces in the flow of life. 



We have some basis for the estimate of the duration of a species. 

 When the great glacial Lake Bonneville occupied the basin of the 

 Great Salt Lake, the same species of fishes and insects were found 

 in all its tributaries. Now that these streams flow separately into a 

 lifeless lake, the same species of fishes occur in them for the most part 

 without alteration. One species of sucker (Catostomus ardens) and 

 one chub (Leuciscus lineatus) are found unaltered throughout this 

 region and in the Upper Snake Biver (above Shoshone Falls), into 



vol. lxviii. — 32. 



