86 SECTIONAL ADDRESSES 



Genetical analysis showed that it differed from melanogaster in having 

 a large section of one chromosome reversed. This must have occurred 

 suddenly, and, once established in homozygous condition, would inhibit 

 the fertility of any heterozygotes. The bar to fertility once established, 

 other differences between the two types could accumulate, though they 

 are still very slight. 



It does not, however, matter in principle whether isolation is effected 

 gradually or abruptly ; in any case subsequent divergence will be gradual 

 (except in some of the cases to be described later, where the isolating 

 process itself produces marked differences in appearance). 



We often know the approximate date at which isolation of an island has 

 occurred, and can see that, broadly speaking, the degree of divergence is 

 proportional to the time that has since elapsed, as well as to the effective- 

 ness of the isolation. It is thus a legitimate deduction that geographical 

 variation provides us with a cross-section of a temporal process, and that 

 isolational divergence has been constantly operative, throughout evolution, 

 as an agency promoting minor systematic diversity. 



The sudden convergent formation of new species as a result of hybridisa- 

 tion has only been established in quite recent years. So far we know of 

 it only in plants. Several cases are known, of which Primula kewensis 

 is the classical example ; but the most striking is that of the rice-grass 

 Spartina Townscndi. This, it now seems certain, is an allopolyploid 

 derived from the crossing of the European S. stricta and the imported 

 S. alterniflora ; most interesting from an evolutionary standpoint is 

 the fact that it is for some reason better equipped than either of its parents, 

 and not only kills them out in competition, but is extending its range 

 beyond theirs. In addition the chromosomal and genetic analysis of 

 various of our most important cultivated plants indicates that they too 

 owe their origin to this process. 



The common existence in plants of species within a genus with different 

 multiples of a basic chromosome-number is also proof of discontinuous 

 species-formation. In some cases this may have been due to autopoly- 

 ploidy, and would be therefore not convergent but divergent. 



The two classical examples of reticulate evolution are the roses and 

 the willows, though similar cases exist in other groups of plants in which 

 species-crossing and chromosome or genome aberrations are prevalent. 

 So far it is not known to exist in animals, except in man. Here it assumes 

 a somewhat different form, since the crossing has been between units of 

 lower than specific rank and no complications of polyploidy, apogamy, 

 and the like have intervened. Thus the result is a single species with 

 a unique degree of variability, in which recombination is the major factor. 

 The evolution of such a group is clearly reticulate. 



Biologists have realised for some time that the term species is loose and 

 difficult of definition. However, whether we can define species or not, or 

 whether we ought to emphasise the distinctions between different kinds of 

 species by refinements of terminology, it remains true that species are 

 genuine biological units . On the other hand , we can distinguish in principle 

 between the causes of their isolation and the causes of their divergence. 

 Groups separated by geographical isolation are species only in posse. 

 Their separation into good species is a slow and subsequent process, 



