168 Robertson: Theories of Evolution. 
created just as we know them now, and have persisted un- 
changed to the present day. The only way to test the value of 
a hypothesis is to ascertain how far it affords an explanation of 
observed facts, and it is on account of its failure to afford such 
an explanation that the doctrine of ‘ Special Creation’ has been 
rejected by biologists as a ‘ working hypothesis.’ On the other 
hand, the theory of Descent with Modification gives a luminous 
conception of the general scheme of organised life. According 
to this theory species are not immutable, but all living beings 
had a common origin somewhere in the remote mists of 
antiquity. The descendants of the original form, or forms 
(which are supposed to have been exceedingly simple in structure) 
have developed with progressive modification along innumerable 
lines, attaining at last to the state of differentiation and com- 
plication which we witness to-day. The theory of Descent 
with Modification gives a real meaning to the existence of 
classificatory units, such as genera, orders, etc., for these are 
regarded as actual expressions of the degree of blood-relation- 
ship ; a genus, for instance, consists of a group of species which 
have a close affinity, being descended from a comparatively 
recent common ancestor. 
The doctrine of Special Creation seems to have been accepted 
almost unquestioningly in ancient and medizval times, though in 
certain classical writings a disbelief in it is vaguely foreshadowed. 
The idea that species are mutable was first definitely expressed 
by Sir Walter Raleigh in the seventeenth century. In his great 
work on ‘The History of the World,’ speaking of the days of 
Noah’s flood, he says, ‘But it is manifest, and undoubtedly 
true, that many of the Speczes, which now seem differing, and 
of severall kindes, were not then z2 rerum natura.’ 
Buffon (1707-1788) was one of the first scientists to clearly 
suggest that species might have been gradually evolved. It is 
not worth while to dwell at length upon his views. He was by 
no means a great naturalist, but he was an amazingly brilliant 
writer ; some of his epigrams, such as ‘Le style c’est homme 
méme,’ live to the present day. He had occasional vivid flashes 
of scientific insight, and was the first to realise that fossils give 
evidence of the existence of extinct animals. Buffon appears to 
have been much afraid of incurring the odium of his orthodox 
contemporaries, and though he sometimes dared to state the 
possibility of Descent with Modification quite clearly, at other 
times he definitely denied such views. According to Professor 
Packard, ‘The impression left on the mind, after reading Buffon, 
Naturalist. 
