1 62 CAUSES OF VARIATION 



All that is involved in fertilization is not well understood, but 

 its essential feature is the union or fusion of tJic nuclear matter 

 (mingling of the chromosomes) from two parents to form the 

 cleavage or segmentation nucleus whose subsequent growtJi and 

 divisions "give rise to all the nuclei of the body." This fertilized 

 ovum becomes, therefore, the first cell of the new being, which 

 inherits directly and equally a portion of the nuclear matter from 

 both parents, so that " every nucleus of the child may contain 

 nuclear substance derived from both parents." * Here, then, is 

 the avenue of all inheritance, and, as the new individual is a kind 

 of blend of both parents, we see in fertilization an initial and 

 primary cause of variation. 



This is the only form of variation recognized by Weismann in 

 his earlier writings as in any sense hereditary. All deviations 

 in development due to external causes were conceived to affect 

 the body (soma plasm 2 ) only, exerting no influence upon the 

 ancestral germ plasm. 3 True, he later announced the theory of 

 germinal selection, in which a kind of struggle for existence is 

 conceived as taking place between the "biophors " (physiological 

 units), by which some prosper and multiply exceedingly while 

 others are crowded out entirely. 4 This would give another cause 

 of variation within the germ plasm of each individual. 



Biologists generally recognize internal causes of variation 

 other than these, and yet this union of the chromosomes from 

 different individuals taking place at each new generation must be 

 regarded as a very effective means of introducing variability. 

 Even if the offspring of a single parent, as in parthenogenesis, 

 should be an exact duplicate of the parent, which it is not, - 

 every one would recognize the fact that the blending of heredi- 

 tary substance from two parents must of necessity produce an 

 individual with a new combination of faculties. 



It is a variation, however, confined not only to the characters 

 of the race but also to the family possessions of the particular 

 parents. Bisexual reproduction cannot be looked upon as a means 



1 Wilson, The Cell, p. 182. 



" Soma plasm " is a term used to represent the protoplasms of the body in 

 general as distinct from the output of the sexual cells (germ plasm). 

 8 Weismann, The Germ Plasm, chap. ix. 

 * Weismann, Germinal Selection (pamphlet). 



