il 



212 OUTLINES OF EVOLUTIONARY BIOLOGY 



coagulated and thereby destroyed as living substance at com- 

 paratively low temperatures, and cannot have'been present on the 

 earth until the surface of the latter had not only solidified but 

 also cooled down to something approaching its present degree 

 of heat. 



Living things must therefore have first put in an appearance 

 at some definite period well advanced in the earth's history 

 According to the biblical account given in the Book of Genesis 

 their advent was due to a series of special acts of creation per- 

 formed by the Creator at the appropriate time, and this account 

 forms the basis of the doctrine of Special Creation which still 

 survives amongst uneducated people. Upholders of this doctrine 

 maintain that each kind of plant or animal was not only created 

 as a separate kind and in a state of full perfection, but was 

 specially designed to suit the conditions under which it was 

 placed, and in this way the marvellous adaptation of plants and 

 animals to their environment was supposed to be accounted for. 

 The fish was created to swim in the sea, the horse to run on 

 dry land, the monkey to climb in the tree and the bird to fly in 

 the air, and each was constructed in accordance with its foreseen 

 requirements. Moreover, each species was supposed to be immut- 

 able, propagating its own kind by reproduction but never changing 

 into a different kind. 1 



Such a view of the origin of living things could only have arisen 

 in a state of almost complete ignorance of the phenomena which 

 have to be accounted for, and at the present day it has been 

 entirely superseded, as a scientific theory, by the doctrine of 

 Organic Evolution. This doctrine teaches us that species or kinds / 

 are not immutable but are subject to changes whereby one mayl 

 give rise to another, and that all existing species are the more or 

 less modified descendants of pre-existing ones. Modern biologists, 

 moreover, maintain that the further back we trace the ancestral 

 history of living things the less diversity of structure do they 

 show, until finally, if we could trace them back to their origin, 

 we should find all the different lines of descent converging towards 

 a common starting point far back in geological time. We shall 

 see presently that the adaptation to environment which forms 

 such a characteristic feature of all living organisms can be 

 explained just as well in accordance with the theory of organic 



1 Compare the quotation from Linnaeus on p. 222 (1st footnote) and that from 

 Buffon on p. 369, 



