218 THE PERPETUATION OF ADAPTATIONS 



represented in the other two (E', E') (Fig. 91, A-E). Fertili- 

 zation may be accomplished by either E or E', and different 

 types of adult will result. There is a similar result with 

 the maturation processes of the egg, where three polar bodies 

 and one egg cell are formed. The eggs resulting will be dif- 

 ferent, according to the disposal of the group of characters 

 during maturation. 



Weismann finds in these phenomena a possibility of the origin 

 of variations which, once originated, may be maintained or 

 exterminated by natural selection. 



The Significance of Pseudo-reduction. The significance of 

 pseudo-reduction has only recently been made out. The 

 diploid number of chromosomes is reduced to the haploid num- 

 ber by union of the chromosomes, two by two. This union is 

 not haphazard, but takes place with remarkable order and pre- 

 cision. It is best shown in those animals in which the chromo- 

 somes are dissimilar in size and shape, as in Anasa tristis (squash 

 bug) , where the twenty-one chromosomes of the male differ in 

 size. It has been demonstrated that these chromosomes occur 

 in two sets of chromosomes, as shown in Fig. 92 (e,f). When 

 union of chromosomes takes place at the period of pseudo- 

 reduction, a, unites with a, b with b, c with c, etc., so that the 

 resultant tetrads are symmetrical. 



These two sets of chromosomes represent the sum total of 

 character factors, received from the two parents at the time of 

 fertilization of the egg, which developed into the adult whose 

 germ plasm we may be studying. Each set of chromosomes 

 contains all of the factors necessary for the complete individual 

 (as shown by parthenogenesis, artificial or normal). Each 

 chromosome represents certain structures of the adult (as 

 determined by experiment), and the union at pseudo-reduction 

 of two similar chromosomes is thought to be the union of the 

 factors having to do with the same characteristics of the adult, 

 one chromosome representing these characteristics in the female 

 parent, the other representing the like character group in the 

 male parent. Hence it follows from the happenings at matura- 

 tion (Fig. 91), if the light area represents certain characteristics 



