EVOLUTION, VARIATION AND HEREDITY 443 



successive change or evolution has come to be one of the most 

 widespread generalisations of modern times and is not limited to 

 the sciences in which it was first propounded, but has now been applied 

 to practically every branch of human thought. We can recognise 

 clearly three distinct sorts of evolution : (i) Inorganic evolution, 

 that is, the formation of the world and the inorganic materials in 

 it ; (2) Organic evolution, that is, the production of the many and 

 varied forms of living beings that fill the world to-day ; and (3) the 

 evolution of things with which man has dealt, such as the growth 

 of languages, religions and human institutions, the development of 

 art, of buildings, of machinery and so on. It is with the second of 

 these, namely, Organic Evolution, that we are practically entirely 

 concerned. 



Two very different things must be clearly borne in mind from the 

 outset : one is that the occurrence of organic evolution is now generally 

 accepted as a fact by all who study even a little biology ; and the 

 other is that there is probably no explanation of it as a process, of 

 the exact causes at work, and the precise manner in which they 

 operate has been generally admitted to be satisfactory. The two 

 things, however, that is, evolution itself and the theories to explain 

 it, must be kept quite distinct, and the failure to do so has led to much 

 confusion of thought, particularly in semi-popular writings, but also 

 in some scientific works. Thus it is by no means uncommon to find 

 in poorly informed quarters a statement to the effect that, in view 

 of the criticisms that have been levelled at Darwin's theory of 

 evolution, evolution itself is no longer believed in by them. Nothing 

 could be further from the truth, for the fact of evolution remains 

 unchallenged and it is only the explanation of the causes at work 

 that is questioned. 



It is desirable, in order to appreciate the more modern 

 views on evolution and some of the theories that have been put 

 to explain it, to examine quite briefly the opinions that were previously 

 held and so approach our present conceptions on the subject from 

 the historical standpoint. 



From the time of the Greeks, who had a very good knowledge of 

 Biology and indeed made some attempts to explain the origin of 

 the diversity of living things, up to the beginning of the eighteenth 

 century, Biological Science was in a chaotic state and in the first 

 part of that period, instead of advancing, fell back. Throughout 

 the Middle Ages little thought was given to the subject, and it was a 

 matter of general belief that there had been a special act of creation 

 and all the animals and plants we now see were created exactly as 

 they are at present. Each particular type was considered fixed 

 and unalterable and spoken of as a species. When fossil remains of 



