PRESENT CRISIS IN EVOLUTIONARY THOUGHT 9 



tion of species due to this indirect influence of a varying en- 

 vironment. 



Such was the theory of Lamarck, and it is sound and 

 plausible in all respects save one, namely, the unwarranted 

 assumption that acquired adaptations are inheritable, since 

 these, to quote the words of the Harvard zoologist, G. H. 

 Parker, ^'are as a matter of fact just the class of changes in 

 favor of the inheritance of which there is the least evidence." 

 ("Biology and Social Problems," 1914, p. 103.) 



The next contribution to the philosophy of transformism 

 was made by Charles Darwin, when, in the year 1859, he pub- 

 lished his celebrated ''Origin of Species." In this work, the 

 English naturalist bases the evolution of organic species upon 

 the assumed spontaneous tendency of organisms to vary 

 minutely from their normal type in every possible direction. 

 This spontaneous variability gives rise to slight variations, 

 some of which are advantageous, others disadvantageous to the 

 organism. The enormous fecundity of organisms multiplies 

 them in excess of the available food supply, and more, accord- 

 ingly, are born than can possibly survive. In the ensuing 

 competition or struggle for existence, individuals favorably 

 modified survive and propagate their kind, those unfavorably 

 modified perish without progeny. This process of elimination 

 Darwin termed natural selection. Only individuals favored by 

 it were privileged to propagate their kind, and thus it hap- 

 pened that these minute variations of a useful character were 

 seized upon and perpetuated ''by the strong principle of 

 inheritance.'' In this way, these ^slight but useful modifica- 

 tions would tend gradually to accumulate from generation to 

 generation in the direction favored by "natural selection," 

 until, by the ensuing summation of innumerable minor dif- 

 ferences verging in the same direction, a major difference 

 would be produced. The end-result would be a progressive 

 divergence of posterity from the common ancestral type, 

 whence they originally sprang, ending in a multiplicity of new 

 forms or species, all differing to a greater or lesser extent from 



