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 “ échelle des étres,” 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. 
