THE ORIGIN OF VARIATIONS 235 



peared in the modern horse. The digestive tract of intestinal 

 parasites shows various degrees of degeneration through disuse. 

 In some of the thread worms (nematodes), it consists in part of 

 a single row of perforated cells; in the tape-worm, it has en- 

 tirely disappeared. 



In order that they may be perpetuated, any changes that may 

 occur in the individual must be represented in the germ plasm. 

 The individual is mainly somatic, and as such is mortal, but he is 

 also the nurse or protector of the germ plasm which is potentially 

 immortal. Whether or not he can impress his individual stamp 

 on the germ plasm, and send it on with the new impress, is the 

 very crux of the problem of species. Modern biology gives no 

 positive solution of this problem. Weismann and his school 

 maintain that all changes are due to re-combinations of factors 

 in the germ plasm; Neo-Lamarckians, that the germ plasm 

 responds directly to the changes in the body, and these in turn 

 go back to the conditions of the external environment. 



The limits of this text-book do not permit an examination of 

 the evidence for and against these two points of view; indeed 

 many important questions which bear upon it have not been 

 even mentioned. Morgan has recently summed up the general 

 problem as follows: "It is true that the germ plasm must some- 

 times change otherwise there could be no evolution. But the 

 evidence that the germ plasm responds directly to the experi- 

 ences of the body has no substantial evidence in its support. I 

 know, of course, that the whole Lamarckian school rests its 

 argument on the assumption that the germ plasm responds to 

 all profound changes in the soma; but despite the very large 

 literature that has grown up dealing with this matter, proof is 

 still lacking. And there is abundant evidence to the contrary. 



"On the other hand there is evidence to show that the germ 

 plasm does sometimes change or is changed. Weismann's 

 attempt to refer all such changes to re-combinations of internal 

 factors in the germ plasm itself has not met with much success. 

 Admitting that new combinations may be brought about in this 

 way, yet it seems unlikely that the entire process of evolution 

 could have resulted by re-combining what already existed ; for 



