214 EVOLUTION IN BIOLOGY vi 



evolution, 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 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 Swammerdam, and continued by the remark- 

 able labours of Reaumur, 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 attention of biologists to the 

 gradation in the complexity of organisation which 

 is presented by living beings, and culminated in 

 the doctrine of the " echelle des etres," so power- 

 fully and clearly stated by Bonnet ; and, before 

 him, adumbrated by Locke and by Leibnitz. In 

 the then state of knowledge, it appeared that all 

 the species of animals and plants could be 

 arranged in one series ; in such a manner that, by 

 insensible gradations, the mineral passed into the 

 plant, the plant into the polype, the polype into 

 the worm, and so, through gradually higher forms 

 of life, to man, at the summit of the animated 

 world. 



