THE VITAL FABRIC OF DESCENT 317 



tion independent of the environment. Evolution must produce 

 characters which the environment can admit, and with unspec- 

 ialized mammalian t3'pes as a beginning, the requirements be- 

 come similar, even though the regions be different. 



With mammals the selective factors are at the very highest, 

 and b}' competing with and preying upon each other they make 

 by far the most effective part of their own environment. The 

 struggle for existence is a stern reality, and the issue rests, very 

 often, on a narrow margin of speed, strength, armament, or en- 

 durance. It need not surprise us, then, that the numerous 

 geological and geographical experiments enumerated by Pro- 

 fessor Osborn have turned out so much the same. Kinetic evo- 

 lution explains the power of radiation, and the selective condi- 

 tions explain the adaptive results, the extent of adaptation being 

 proportional to the thoroughness of the selection, providing of 

 course, that the group be not narrowed to the point of degenera- 

 tion. The most specialized types have ever been the most liable 

 to extinction. 



INADEQUATE MECHANICAL CONCEPTIONS OF HEREDITY. 



The prepotency of symbasic wild types and the " reversion" 

 of domesticated varieties when selective inbreeding is relaxed, 

 are manifestations of the biological laws of which mutation and 

 Mendelism represent the violations. The problems are histori- 

 cal rather than mechanical ; to interpret the facts in terms of 

 descent rather than in those of crudely inadequate and wholly 

 hypothetical " hereditary mechanisms." The organism may be 

 described, for some purposes, as a machine, but it is no mere 

 corn-sheller or steam engine, and there is no assurance that we 

 have, as yet, even a basis of conjecture regarding the principles 

 on which it is constructed, or the ultimate nature of the materials 

 of which it is made. What the mechanism does, however, is a 

 very practical and pressing question which need not be post- 

 poned on account of any lack of agreement in general theories, 

 if, indeed, the workings of the device do not afford the best 

 clue to an understanding of its structure. 



The formal recognition of gravitation and other natural laws 

 or properties has proved useful, although mechanical explana- 



