xvm SUMMAEY 651 



is twofold: first, the difficulty of framing any other theory of 

 recapitulation seems to be insuperable; and, second, the experiments 

 which have been held to disprove the inheritance of acquired char- 

 acters are far from conclusive. 



To take the first point first : If the chequered course of develop- 

 mental history is the result of the preservation of chance variations 

 in the struggle for existence, then we have to assume that, in the 

 eggs of animals living in one environment, variations occurred 

 which were suitable to new environments, and which manifested 

 themselves at the period of adolescence. That, to take a concrete 

 example, among the eggs of the Copepod ancestor of Adheres, some 

 varied so that at adolescence the organisms developed from them 

 tended to lose all their appendages, and that this loss had nothing 

 to do with the reaction to the new environment (gills of fish) in 

 which the animals found themselves. Or, to take another instance, 

 the hermit crab did not acquire its curved abdomen in consequence 

 of the habit of thrusting it into the empty shells of gastropod 

 molluscs; but since the curvature appears in these animals when they 

 are reared in confinement from the larval stage and prevented from 

 finding shells at all (Thomson, 1904), we must assume that in this 

 species the tail became curved in the proper spiral by chance varia- 

 tions, and that then its possessor formed the useful habit of seeking 

 gastropod shells to clothe it. Such explanations are perhaps not im- 

 possible, but, to speak frankly, they do not commend themselves 

 to us. 



To consider now the second objection. The evidence from 

 experiments is not at all' conclusive. The earlier experiments 

 designed to test the inheritability of acquired characters, can only 

 be described as childish. They consisted in such things as cutting 

 off the tails of mice, and in rearing the offspring of these mutilated 

 animals, in the expectation that the young would be born without tails. 

 If anything acquired can be inherited it must surely be some reaction 

 of the organism as a whole. Now Kammerer (1913) has shown that, 

 by keeping salamanders for several generations in certain environ- 

 ments marked changes in skin coloration result ; and that, after a 

 ■ time, not only do the beginnings of these changes show themselves 

 in the young before they are exposed to the new environment, but 

 these altered forms, when crossed with the original forms, behave 

 like Mendelian races (see Chapter I.). 



It must be remembered that the experiments of the Mendelian 

 school of biologists, which are held to demonstrate the unalterable 

 character of the reproductive substance or germ-plasm, have been 

 continued through only a very few generations; whilst to obtain 

 the inheritance of acquired characters the action of the new environ- 

 ment would probably, in most cases, be continued through thousands 

 of generations. The evidence from I'alaeontology seems to suggest 

 that, in making structural advances, Nature acts with ex;treme 

 slowness. It required the whole length of the Secondary Period to 



