THE BREAKDOWN OF CONTINUITY 



small. In so far as it is successful, however, it will grow until its 

 own inlierent instability once again becomes too great and new 

 restrictions on recombination arise within it to give a further 

 break-up of the continuum (Fig. 8i). 



This course of events is well illustrated by the three Drosophila 

 species which we have already had cause to discuss. These are 

 distinguished simultaneously by differences in chromosome sequence 

 and in breeding behaviour, the differences being less between the 

 more recently separated pseudoohscura and persimilis than between 

 these taken together and miranda. Although they wiU cross, they 

 prefer to mate within the species. Their hybrids are both cyto- 

 logically erratic and genetically inadequate. Evidently their genetic 

 architectures are different and the exchange of genes is at least 

 largely prevented between them. In consequence they can overlap 

 in territory without loss either of fitness or of specific distinction. 

 It would appear that miranda has been the least successful, and 

 pseudoohscura the most successful, derivative of their ancestral mating 

 continuum. D. pseudoohscura, in particular, has become widespread 

 and is now showing signs suggestive of instability. Itself one of 

 the products of splitting in past continua, it appears hkely to be 

 on the verge of another sphtting. 



Summing up, it appears that the only universal and permanent 

 type of species is that which allows of regular outbreeding. This 

 type of species is a group of individuals held together in a 

 continuum by genetic recombination. Any change leading to the 

 restriction or breakdown of this recombination leads to discon- 

 tinuity, or the origin of new species. Whether the first step m the 

 restriction or breakdown is external, e.g. geographical or ecological, 

 or whether it is internal, e.g. structural or numerical change of the 

 chromosomes, a bar to crossing wiU be estabhshed which wiU bring 

 aU other forces favouring isolation into action. There is yet another 

 type of internal change, a gene or chromosome change which 

 establishes obligatory inbreeding or apomixis, which in itself breaks 

 up the continuum into its individual items. In so doing it produces 

 species which, however, are unlike the parental outbreeding type 

 and have no evolutionary future. 



320 



