80 ON HEREDITY. 



reproductive cells would continue to appear, sntr-'fcheeew-ouldbe 

 increasecLand- rendered, permanent _by__jneaiis fifjaaimaL selection, 

 when their results, in the alteration of certain cells in the body, 

 were advantageous to the species. The only condition necessary 

 for the transmission of such changes is that a part jjf _the__rej>ro- 

 ductive substance (the germ-plasm) should always remain nnnhanggd 

 lurtng^segmentation and the subsequent building up of the body, 

 or in other words, that such unchanged substance should pass into 

 the organism, and after the lapse of a variable period, should reappear 

 as the reproductive cells. Only in this way can we render to some 

 extent intelligible the transmission of those changes which have 

 arisen in the phylogeny of the species ; only thus can we imagine 

 the manner in which the first somatic cells gradually developed 

 in numbers and in complexity. 



It is only by supposing that these changes arose from molecular 

 alterations in the reproductive cell that we can understand how the 

 reproductive cells of the next generation can originate the same 

 changes in the cells which are developed from them ; and it is 

 'impossible to imagine any way in which the transmission of changes, 

 produced by the direct action of external forces upon the somatic 

 cells, can be brought about l . 



The difficulty or the impossibility of rendering the transmission 

 of acquired characters intelligible by an appeal to any known force 

 has been often felt, but no one has hitherto attempted to cast doubts 

 upon the very existence of such a form of heredity. 



There are two reasons for this : first, observations have been 

 recorded which appear to prove the existence of such transmission ; 

 and secondly, it has seemed impossible to do without the supposition 

 of the transmission of acquired characters, because it has always 

 played such an important part in the explanation of the trans- 

 formation of species. 



It is perfectly right to defer an explanation, and to hesitate 



1 To this class of phenomena of course belong those acts of will which call forth 

 the functional activity of certain groups of cells. It is quite clear that such im- 

 pulses do not originate in the constitution of the tissue in question, but are due to the 

 operation of external causes. The activity does not arise directly from any natural 

 disposition of the germ, but is the result of accidental external impressions. A 

 domesticated duck uses its legs in a different manner from, and more frequently than 

 a wild duck, but such functional changes are the consequence of changed external 

 conditions, and are not due to the constitution of the germ. 



