THEOKKTKAI. BIOLOGY 



477 



In the embryology of many animals it is possible to Nil low the 

 development of the germ-cells of the adult directly from the 

 original germ-cell, the fertilized ovum. The somatic cells de- 

 velop from it, but a part, destined to become the germ-cells of 

 the adult, is unaltered by the development of the body about it. 

 There is thus a direct continuity of germ substance, of germ 

 plasm from parent to offspring, from the very beginning ol the 

 race. Hence it is not surprising that the fundamental char- 

 acteristics of the species are inherited from generation to 

 generation. 



But in order that there may be any evolution there must be 

 an inheritance of variations ; these may lie either in the germ- 

 cells themselves, as expressed in Weismann's theory, or they 

 may be derived from the parent through the use of its various 

 organs. It is extremely difficult to get any evidence of fact 

 which shall speak conclusively for the one supposition or the 

 other. It is very necessary, too, to distinguish between a real 

 inheritance and the reappearance of characters which may be 

 due merely to similar conditions of life. Many biologists 

 believe, with Weismann, that there is absolutely no inheritance 

 of acquired characters; many believe, with Lamarck, that evo- 

 lution is brought about only by the inheritance of acquired 

 characters ; and many are inclined to take the middle course, 

 which was also held by Darwin, that while natural selection is 

 the primary factor in evolution, there may be an inheritance of 

 characters acquired during the life of the individual. 



Theories have been suggested to account for the way in which 

 the somatic cells must affect the germ-cells if acquired charac 

 ters are to be inherited, but none of them has found any gen- 

 eral acceptance among scientists, since they have had no tacts 

 of weight behind them. Darwin's own theory of pangenesis, 

 which he qualified as provisional, assumed the transmission of 

 minute particles, which he called gemmules, from every cell in 

 the organism to the germ-cells, which were thus able to bear 

 them as an inheritance to the offspring, where they developed 

 into the same kind of cells as existed in the parent; there is no 

 evidence that such gemmules exist, and many facts are against 

 their existence; to-day the theory is chiefly of historic interest. 

 That the removal or lack of development of the germ-cells often 



