THE FACTORS OF EVOLUTION 185 



But let us return to Darwin's theory of selection. We have 

 seen that natural selection alone can supply no satisfactory 

 explanation of the evolution of the organic world, that its value 

 in the first heat of enthusiasm has been greatly exaggerated, and 

 that it is in no case possible to prove that the struggle for exist- 

 ence does perform the role of the breeder. Hence many natural- 

 ists have now arrived at the conclusion that the part played by 

 the struggle for existence is not so much one of selection, but 

 rather tends to exterminate inferior individuals, and to exclude 

 them from the reproductive function. But here, again, we must 

 be careful not to exaggerate the value of elimination, for 

 numerous cases are known of cripples with serious pathological 

 defects who have been able to live many years in spite of this 

 supposed fierce struggle. For instance, Pauly describes a pike, 

 about 50 cm. long, with a malformation of the upper jaw, giving 

 the appearance as if this had been cut off 2 or 3 cm. from the 

 point. But though the fish was greatly hampered in its chase 

 after prey, it was yet well able to maintain itself until chance 

 brought it into the fisher's net. Hofer mentions a case of a 

 two-year-old carp whose mouth-aperture was grown together in 

 such a manner that the fish was forced to take its food through 

 the gill-clefts. 



But if the theory of selection does not offer a satisfactory 

 explanation, how are we to conceive the process of the origin of 

 species? We have already heard how Lamarck attempted to 

 overcome these difficulties. His explanation seems clear, even 

 if it does not suffice for all phenomena, in particular for the 

 acquisition of passive adaptations. It is, however, based to some 

 extent upon a factor which has not yet been proved, the heredity 

 of acquired characters. 



If we consult the scientific literature upon this point we find 

 widely divergent views. While some writers flatly deny the 

 possibility of acquired characters, others equally emphatically 

 support it. Let us, therefore, consider the facts so that we may 

 form our own conclusions. 



In order to demonstrate the impossibility of such transmission 

 some people cut off the tails of mice, repeating this process 

 during several succceeding generations ; and as the descendants 

 of these tailless mice were nevertheless always born with fully 



