134 FLEAS, FLUKES AND CUCKOOS 



Convergent and Parallel Evolution. The case just described is an example 

 of convergent evolution. That is to say, a louse which is not closely 

 related by recent common ancestry to other head lice has assumed 

 similar characters in response to the same environment. Their resem- 

 blance thus indicates a similar history rather than a similar ancestry. 

 Problems caused by convergent evolution may be responsible for many 

 of the difiiculties in the classification of the Mallophaga. Sometimes it 

 is relatively easy to unravel these relationships, but if sufficient time has 

 elapsed to enable the new occupant to adapt itself along closely similar 

 lines so that it comes to resemble the original inhabitant of the niche, 

 mistakes can easily be made. It is consequently difficult to decide 

 which characters in the louse indicate relationship or derivation from a 

 common ancestor, and which are developed as a result of living in the 

 same environment. On many birds the lice belonging to different 

 genera will show a number of similar characters. Some of these, without 

 doubt, are developed as a response to the type of feathers forming the 

 plumage of the host. The lice living on birds with iridescent feathers, 

 for instance, sometimes have a thicker exoskeleton with a sculptured or 

 pitted surface. 



Another factor which may be responsible for some of the present 

 confusion in the classification of the Mallophaga is so-called parallel 

 evolution. This term is used to describe a case where two primitive 

 stocks office, after diverging, have evolved along similar lines. Parallel 

 evolution is, therefore, the independent acquisition by related groups of 

 similar characters during their evolution. In contradistinction, con- 

 vergent evolution is the acquisition of similar characters by unrelated 

 groups in response to a similar habitat — the whales and the fish being a 

 well known example. It is often difficult for the parasitologist to decide 

 whether two groups of species are strikingly alike because they are close 

 relatives or whether the likeness is due to parallel evolution. 



Discontinuous Distribution. Discontinuous distribution is a term used 

 to describe populations of animal species which are divided from one 

 another by large geographical areas in which their own kind is totally 

 absent. It is believed that many animals which once had a continuous 

 range over a wide geographical area, such as Europe, have become 

 extinct in parts of this range, leaving isolated populations here and 

 there. Since isolation is an important factor in species formation (see 

 p. 1 38) these animals may evolve into new species or even new genera. 



