THE DEVELOPMENT OP VERTEBRATA. 287 



of successive generations may be compared to that of 

 successive leaves on a growing shoot. Each leaf is not 

 produced by the preceding leaf, but owes its resemblance 

 to the latter to the fact that it is produced by the same 

 shoot. The gametocytes are continued by cell division 

 from generation to generation, the individuals are out- 

 growths from the line of gametocytes. 



10. Blastogenic Origin of Variations. It was formerly 

 assumed that modifications of the soma by external con- 

 ditions and by use and disuse were inherited in some 

 slight degree, so that in each generation the amount of 

 modification was increased, and in this way adaptations 

 might be produced. Since the time of Weissman, as the 

 above distinction between soma and gametocytes has been 

 more thoroughly realised, the majority of biologists have 

 adopted the view that changes in the soma do not affect 

 the gametocytes, in other words are not inherited, and that 

 all variations arise in the gametocytes, or are blastogenic. 



11. Minute "Continuous" Variations. That blasto- 

 genic variations frequently occur is certain, but the study 

 of variation presents many difficulties and has been at- 

 tempted from different points of view. A distinction 

 must be made between the minute differences which 

 always exist between a number of individuals of the 

 same species, and even between the offspring of the same 

 parents, and those definite and conspicuous variations 

 which are not connected with the normal type by inter- 

 mediate degrees. For the latter the term mutation, 

 introduced by the Dutch naturalist de Vries, has been 

 adopted. The minute individual differences were those 

 on which Darwin chiefly relied for his theory of Natural 

 Selection, and they have been investigated statistically by 

 Galton, Weldon, and Karl Pearson, who maintain Darwin's 

 original view. 



Many objections have been raised to this view : one is 

 that the differences are too small to determine death or 

 survival ; another that it is not certain that such differences 

 are inherited, as many of them may be due to external 



