154 CONCLUSIONS 



In effect, much of the evolution of the major groups of animals 

 has to be taken on trust. There is a certain amount of circum- 

 stantial evidence but much of it can be argued either way. Where, 

 then, can we find more definite evidence for evolution? Such 

 evidence will be found in the study of modern living forms. It 

 will be remembered that Darwin called his book The Origin of 

 Species not The Origin of Phyla and it is in the origin and study of 

 the species that we find the most definite evidence for the evolution 

 and changing of form. Thus to take a specific example, the 

 Herring Gull, Larus argentatus, does not interbreed with the 

 Lesser Black-backed Gull, Larus fuscus, in Western Europe, the 

 two being separate species. But if we trace L. argentatus across 

 the northern hemisphere through North America, Eastern Siberia 

 and Western Siberia we find that in Western Siberia there is a form 

 of L. argentatus that will interbreed with L. fuscus. We have here 

 an example of a ring species in which the members at the ends of 

 the ring will not interbreed whilst those in the middle can. The 

 separation of what was possibly one species has been going on 

 for some time (in this case it is suggested since the Ice Age). 

 We have of course to decide that this is a case of one species 

 splitting into two and not of two species merging into one, but 

 this decision is aided by the study of other examples such as those 

 of small mammals isolated on islands, or the development of 

 melanic forms in moths. Details of the various types of speciation 

 can be found in the books by Mayr, Systematics and the Origin 

 of Species (1942), and Dobzhansky, Genetics and the Origin of 

 Species (1951). 



It might be suggested that if it is possible to show that the 

 present-day forms are changing and the evolution is occurring 

 at this level, why can't one extrapolate and say that this in effect 

 has led to the changes we have seen right from the Viruses to the 

 Mammals? Of course one can say that the small observable 

 changes in modern species may be the sort of thing that lead to 

 all the major changes, but what right have we to make such an 

 extrapolation? We may feel that this is the answer to the problem, 

 but is it a satisfactory answer? A blind acceptance of such a view 

 may in fact be the closing of our eyes to as yet undiscovered factors 

 which may remain undiscovered for many years if we believe that 

 the answer has already been found. 



