EVOLUTION IN BIOLOGY. 305 



f essor Haeckel * 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 " Principes 

 de la Philosophic " 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, respect- 

 ively, have a clear significance on the hypothesis of evo- 

 lution, while they are unintelligible if that hypothesis be 

 denied. And those of the eighth group are not only 

 unintelligible without the assumption of evolution, but 

 can be proved never to be discordant with that hypothe- 

 sis, while, in some cases, they are exactly such as the 

 hypothesis requires. The demonstration of these asser- 

 tions would require a volume, but the general nature of 

 the evidence on which they rest may be briefly indicated. 



2. The accurate investigation of the lowest forms of 



* " Gcnerclle Morphologic," 1866. 



