484 AN INTRODUCTION TO ZOOLOGY 



followed this process of " Divergent Evolution." They tend to 

 diverge more and more from one another and from the ancestral 

 stock as evolution proceeds, and we can express their inter-relations 

 schematically in a branched fan-like diagram. 



Looked at from another point of view, we may regard it as an 

 illustration of what has been termed " Adaptive Radiation."" That 

 is to say, when a form or type of animal organisation has been 

 established, its descendants proceed to spread out and become 

 adapted to a number of different environments : the air, the sea, 

 the swamp, the forest, the arid plain, and so on. Any line may 

 become highly adaptive, possessing remarkable peculiarities, yet 

 each group retains certain characters, not the same in each case, of 

 course, reminiscent of the ancestral form. 



Another phenomenon not nearly so widespread as divergence, but, 

 nevertheless, of considerable importance, may also be noted. It 

 follows as a result from this adaptive radiation that takes place in 

 various groups. Members of various and quite different groups 

 may take to living in almost identical environments, say, for example, 

 the air or the sea. If it is an environment that demands a high 

 degree of specialisation in order to bring about satisfactory adapta- 

 tion, as both of these do, then we sometimes find animals of quite 

 different groups assuming forms with a greater or less amount of 

 superficial resemblance. Thus, for example, we find birds, bats, 

 and pterodactyls, while not at all closely related, live in the same 

 environment and exhibit obvious similarities, particularly the last 

 two. Again, there is a strong resemblance between Fish, Ichthyo- 

 saurs, Mosasaurs, Dolphins, and Whales, and in the case of the limb- 

 less lizards, e.g. Blind-worms and Snakes, the approach is very 

 striking. To this we apply the term " Convergent Evolution " or 

 " Parallelism in Evolution." In classifying animals it is obvious 

 that this convergence must be taken into account, or otherwise we 

 should be liable to group together erroneously animals whose 

 similarities are due to their being adapted to a similar environment, 

 and not to a community of descent. 



Here the above outline of some of the main problems that 

 arise from a consideration of Evolution, Variation, and Heredity 

 may be brought to a conclusion. It is not intended to imply that 

 there are no further problems to be studied, but this chapter does 

 not aim at being even a statement of all the various biological 

 theories, which would in itself require a volume many times larger 

 than this. Rather has it attempted to indicate certain outstanding 

 points and to provide an introduction to further and wider reading 

 in these matters in which many excellent texts are now available. 



