D.— ZOOLOGY 85 



become transformed gradually until it comes to merit a new specific 

 name. Or it may separate, also gradually, into two or more divergent lines. 

 Sometimes the separation into mutually infertile groups may occur 

 suddenly, but the subsequent divergence may yet be gradual. Or it may 

 hybridise with another species and the hybrid product then, by doubling 

 of the chromosomes (allopolyploidy), give rise at one bound to a new 

 species. Here, instead of one species diverging to form two, two converge 

 to form one. (It is possible that such sudden origins of new species by 

 means of chromosome or genome aberrations may also occur without 

 hybridisation, from a single instead of a dual origin.) Finally, in certain 

 groups of plants, the minor systematics are in an inextricable tangle, so 

 that no two authorities agree even approximately as to the number of 

 species involved and their limitations ; in these cases hybridisation, 

 apparently involving many more than two forms, together with recom- 

 bination, chromosome-doubling, and apogamy, appears to have been and 

 still to be at work. Thus species-formation may be continuous and uni- 

 linear ; continuous and divergent ; abiupt and convergent ; or what, 

 following a recent writer, we may call reticulate, dependent on constant 

 intercrossing and recombination between a number of lines, and thus 

 both convergent and divergent at once. 



Palaeontology provides numerous evidences of gradual specific trans- 

 formation ; these have been preserved almost exclusively in aquatic 

 animals, though also in a few land vertebrates such as the horses ; but 

 similar changes must, it is clear, have been generally at work. In some 

 cases at least, as in the shift of the mouth in the sea-urchin Micraster, 

 the change seems to have been an adaptive improvement. 



Divergent splitting must clearly be postulated on a large scale, if only 

 to account for the rapid increase of the number of forms in newly evolved 

 groups such as the higher placental orders. It is not easy to obtain direct 

 evidence of divergence from palaeontology, since this demands good series 

 in two separate but crucial areas. But what without question are different 

 stages of the process are yielded by a study of geographical distribution. 

 This reveals all stages of geographical divergence, from dubious to sharply 

 defined subspecies, and thence on to species and genera. 



Physiological subspecies, such as the races of gall-forming insects 

 restricted to different host plants, are of a similar nature, though their 

 distinctive characters are not among those which appeal to the museum 

 systematist. 



In all these cases isolation, whether geographical or physiological, 

 is involved. Although sometimes, as in many of the geographical colour- 

 varieties of the mouse Peromyscus, the differences seem definitely to be 

 adaptive, in others subspecific distinctions appear as biologically meaning- 

 less, as do many specific differences between allied species. We cannot 

 be sure whether isolation simply makes it easier for selection to cause 

 adaptive divergence in relation to local conditions, or whether in some 

 cases at least, by some method as yet obscure, it permits the fruition of 

 mere random and biologically useless variation. 



An interesting case in which we must presume the isolation to have been 

 suddenly effective is that of Drosophila simulans, which so closely resembles 

 D. melanogaster that it was wholly overlooked by the systematists. 



