350 THE VARIATION OF ANIMALS IN NATURE 



presented by the simultaneous occurrence of specialised and 

 unspecialised (or archaic) forms. His main points are as 

 follows : Primitive forms may have survived unmodified, be- 

 cause (i) no beneficial variations occurred, (2) they are not 

 really competing with ' higher ' forms, (3) unknown factors 

 may have been at work. Alternatively, they may actually 

 be highly evolved compared with their past state, or they may 

 more recently have suffered retrogression. 



To us these arguments do not appear to touch the central 

 point at issue. We can often see the value of some specialisa- 

 tion after the first steps in that direction have been taken, 

 but it is the first steps that require explanation. Thus, in 

 the solitary bees, if a species began to restrict its breeding 

 season to a short period, it might be advantageous to visit 

 only one species of flower which was then abundantly in 

 bloom ; or, conversely, if a bee specialised more and more 

 in visiting one species of flower, a close phenological corre- 

 spondence would be desirable. But we cannot explain why 

 the initial specialisation began except by an appeal to ignor- 

 ance, assuming either an unknown advantage or a hypo- 

 thetical environmental stress. The appeal to ignorance might 

 legitimately be used (with caution) in an endeavour to 

 eliminate the difficulties raised by some thoroughly tested 

 theory, but it cannot safely be used to manufacture the evi- 

 dence on which to a large extent the theory is based. 



We may also examine the use of the word ' adaptable.' 

 An adaptable species is, in normal usage, one which is able 

 to exist in a wide range of conditions. Grinnell and Swarth 

 (191 3, p. 394) include also the power of so existing without 

 marked changes in specific characters. Such ' adaptable ' 

 species may be contrasted with what vertebrate taxonomists 

 usually call ' adapted ' species, i.e. those limited to small, 

 well-defined areas and often showing conformity (especially 

 in colour) with some special feature of the habitat. Doubtless 

 the ' adapted ' species are more specialised, and they may be 

 more closely adjusted to their limited environment, but it is 

 probable that the ' adaptable ' species will leave more 

 descendants. Specialisation is not a passport to succeeding 

 geological periods, though it may lead temporarily to large- 

 scale ' speciation.' It would, indeed, be possible to construct 

 an evolutionary theory which ascribed most of the division 

 of the animal kingdom into species to the action of Natural 



