CO-OPERATION OF FACTORS 405 



Even the individual can do much in its own lifetime to adapt 

 itself to its environment, and when the residua of all the 

 individual adaptations are summed up by inheritance the result 

 is such that we may well wonder how it can have been produced. 

 Throughout the whole process, of course, natural selection must 

 help by constantly weeding out inferiority, but it is probably 

 the direct influence of the environment, including the use and 

 disuse of organs in response to that influence, that is in most 

 cases the determining factor in bringing about adaptation. 



It may well be, however, that there are also cases in which 

 natural selection alone, acting through the occurrence of purely 

 fortuitous variations, has, in the struggle for existence, been 

 sufficient to produce marvellous adaptations. This may have 

 been the case with protective resemblance and mimicry in form 

 and colour, and with the adaptation of flowers for fertilization by 

 insects, in all of which it is difficult to see how the direct action 

 of the environment or the use and disuse of organs could bring 

 about adaptive modifications. But the fact that natural selection 

 alone appears to have been sufficient in some cases must not 

 prevent us from admitting the action of other factors in other 

 cases. The fact that some carriages are pulled by motors affords 

 no justification for asserting that other carriages may not be 

 pulled by horses, or that the same carriage may not at one 

 time be pulled by a motor and at another by a horse, or even 

 by both together. Many factors must have co-operated to 

 bring about such a marvellously complex result as the present 

 condition of the organic world, and no sufficient reason has 

 yet been shown for denying ourselves the assistance of 

 " Lamarckian " factors in our endeavours to discover the processes 

 through which this result has come about. 1 



We may now turn our attention to a group of cases which 

 certainly appear to support the view that the factors at work in 

 determining any particular line of evolution are more complex 

 than might at first sight be supposed. It is a fact well known 

 to palaeontologists that many widely separated groups of the 

 animal kingdom have, during the course of their evolution, and 

 especially towards the end of that course, shown a strongly 

 marked tendency to enormous increase in size. We see this 

 in the extinct eurypterids (Fig. 137), giants amongst the 



1 " To insist on ascribing complex results to single causes is the well-known vice 

 of narrow and untrained minds " (Morley's " Life of Gladstone." Vol. II., p. 68). 



