_- 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 4 
and Professor Haeckel 2 have dealt with the whole 
, fEsroblem 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 “Principes de la 
_ Philosophie” 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-r eaching 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. 
* Generelle Morphologie, 1866. 
La nor 
