28 GERMINAL SELECTION. 



an eye were presented, and were presented further ex- 

 actly at spots at which organs of vision could perform 

 their best work : thus, in Turbellaria and many other 

 worms that live in the light, at the anterior extremity 

 of the body and on the dorsal surface; in certain 

 mussels, on the edge of the mantle ; in terrestrial snails, 

 on the antennae; in certain tropical marine snails in- 

 habiting shallow waters, on the back; and in the 

 chitons eveti on the dorsal surface of the shell ! 



But even taking the very simplest cases of selection., 

 it is impossible to do without this assumption, that 

 the useful variations are always present, or thatjA^j 

 (^&yj ?£^li in a suffici ently la rj:.e^ number of individ- 

 uals for the , selective process. You know the thick- 

 ness and power of resistance of the egg-shells of 

 round-worms. The eggs of the round-worms of 

 horses hive been known to continue their course of 

 development undisturbed even after they had been 

 thrown into strong alcohol and all other kinds of in- 

 jurious liquids — ^much to the vexation of the embry- 

 ologists, who wished to preserve a definite stage of de- 

 velopment and sought to kill the embryo at that stage. 

 Indeed, think of the result, if in the course of their 

 phylogenesis stout and resistant variations of egg- 

 shells had not been presented in these worms, or had 

 not always been presented, or had not been presented 

 in every generation and not in sufficient quantities. 



The cogency of the facts is absolutely overpowering 

 when we consider that practically no modification 

 occurs alone, that every primary modification brings 

 in its train secondary ones, and that these induce 

 forced modifications in many parts of the body, fre- 

 quently of the most diversified, or even self-contra- 

 dictory, forms. Recently Herbert Spencer has drawn 



