436 weismann's theory of heredity. 



summation a more and more pronounced tendency, till eventually the 

 acquired character might be as strongly inherited as any other charac- 

 ter which was ah initio congenital. Now it is the validity of this 

 assumptiou that is challenged by Professor Weismann. He says there 

 is no evidence at all of any acquired characters being in any degree in- 

 herited, and therefore that in this important respect they may be 

 held to differ from congenital characters in kind. On the supposition 

 that they do thus differ in kind, he furnishes a very attractive theory 

 of heredity, which serves at once to explain the dilierence, and to rep- 

 resent it as a matter of physiological impossibility that any acquired 

 character can, under any circumstances whatsoever, be transmitted to 

 progeny. 



In order fully to comprehend this theory, it is desirable first of all to 

 explain Professor AVeismaun's views upon certain other topics which 

 are more or less closely allied to, and indeed logically bound up with 

 the i)resent one. 



Starting from the fact that uni-cellular organisms multiply by fission 

 and gemmation, he argues that aboriginally and potentially, life is 

 immortal ; for, when a protozoon divides into two — more or less equal 

 parts by fission, and each of the two halves thereupon grows into 

 another protozoon, it is evident that there has been no death on the 

 part of any of the living material involved ; and inasmuch as this 

 process of fission goes on continuously from generation to generation, 

 there is never any death on the part of such protoplasmic material, 

 although there is a continuous addition to it as the numbers of individ- 

 uals increase. Similarly, in the case of gemmation, when a protozoon 

 parts with a small portion of its living material in the form of a bud, 

 this portion does not die, but develops into a new individual; and 

 therefore the process is exactly analogous to that of fission, save that 

 only a small instead of a large part of the parent substance is involved. 

 Now if life be thus immortal in the case of uni-cellular organisms, why 

 should it have ceased to be so in the case of multicellular organisms ? 

 Weismann's answer is that all the multi-cellular organisms propagate 

 themselves, not exclusively by fission or gemmation, but by sexual fer- 

 tilization, where the condition to a new organism arising is — that minute 

 and specialized portions of two parent organisms should fuse together. 

 Now it is evident that with this change in the method of propagation, 

 serious disadvantage would accrue to any species if its sexual individ- 

 uals were to continue to be immortal ; for in that case every species 

 which multiplies by sexual methods would in time become composed of 

 iudivuals broken down and decrepit through the results of accident 

 and disease — always operating and ever accumulating throughout the 

 course of their immortal lives. Consequently as soon as sexual methods 

 of propagation superseded the more primitive asexual methods, it 

 became desirable in the interests of the sexuallj^ -propagating si)ecies 

 that their constituent individuals should cease to be immortal, so that 



