VI EVOLUTION IN BIOLOGY 213 



important contributions to the solution of the 

 problem of biological evolution, Mr. Darwin con 

 fines himself to the discussion of the causes which 

 have brought about the present condition of living 

 matter, assuming such matter to have once come 

 into existence. On the other hand, Mr. Spencer 1 

 and Professor Haeckel 2 have dealt with the whole 

 problem of evolution. The profound and vigorous 

 writings of Mr. Spencer embody the spirit of 

 Descartes in the knowledge of our own day, and 

 may be regarded as the &quot; Principes de la 

 Philosophic &quot; of the nineteenth century ; while, 

 whatever hesitation may not unfrequently be felt 

 by less daring minds, in following Haeckel in many 

 of his speculations, his attempt to systematise the 

 doctrine of evolution and to exhibit its influence 

 as the central thought of modern biology, cannot 

 fail to have a far-reaching influence on the progress 

 of science. 



If we seek for the reason of the difference 

 between the scientific position of the doctrine of 

 evolution a century ago, and that which it occupies 

 now, we shall find it in the great accumulation 

 of facts, the several classes of which have been 

 enumerated above, under the second to the eighth 

 heads. For those which are grouped under the 

 second to the seventh of these classes, respectively, 

 have a clear significance on the hypothesis of 



1 First Principles, and Principles of Biology, 1860-1864. 



2 Oenerelle Aforphologie, 1866. 



