94 THE CHROMOSOMES 



Oenothera produce trisomic gametes fairly often as a 

 result of one chromosome in the ring going to the 

 wrong pole at the first meiotic division. They also 

 produce new types of gametes as a result of occasional 

 chiasma-fotmation in the median segments. The 



* mutants ' of De Vries (on which he based his 



* Mutationstheorie ') were not really due to genet- 

 ical mutation, but to occasional cross-overs of this 

 type. 



Origest of New Species in Bisexual 

 Organisms 



In animals (with the exception of a few partheno- 

 genetic and hermaphrodite groups such as the Pul- 

 monata, Oligochaeta and in some Lepidoptera and 

 Crustacea) polyploidy is entirely absent and conse- 

 quently cannot account for the origin of new species. 

 On the other hand the variation in chromosome 

 number is almost as great in animals as in plants 

 (cf. Tables VIII and IX). It is therefore clear that 

 various mechanisms exist whereby chromosome num- 

 bers can be altered, and it is probable that these are 

 closely connected with the origin of new species in 

 some cases. It used to be supposed ^^' that two or 

 more chromosomes could merely fuse together to form 

 a single one, and alternatively that one chromosome 

 could break into a number of pieces, each of which 

 would behave in future as a separate chromosome. If 

 this were so a study of chromosome numbers would 

 be of little importance. We now know, however, that 

 each chromosome contains a single spindle attachment 

 which is a self-perpetuating body ; new spindle 

 attachments only arise from pre-existing ones.^^^ 

 Moreover, although spindle attachments divide longi- 

 tudinally at mitosis, they do not appear to be trans- 

 versely divisible. It is possible that in some cases 

 V-shaped chromosomes have two spindle attach- 

 ments in the middle only separated by a very short 



