298 EVOLUTION IN BIOLOGY. [LECT- 



Philosophie " of the nineteenth century ; while, what- 

 ever 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 evolution, while they are unintelligible 

 if that hypothesis be denied. And those of the eighth 

 group are not only unintelligible without the assump- 

 tion of evolution, but can be proved never to be dis- 

 cordant with that hypothesis, while, in some cases, 

 they are exactly such as the hypothesis requires. The 

 demonstration of these assertions 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 animal life, commenced by Leeuwenhoek and Swam- 

 merdam, and continued by the remarkable labours 

 of Eeaumur, Trembley, Bonnet, and a host of other 

 observers, in the latter part of the seventeenth and the 

 first half of the eighteenth centuries, drew the atten- 

 tion of biologists to the gradation in the complexity of 



