CHAPTER I, 
CLASSIFICATION, 
THE first line of direct evidence in favour of organic 
evolution which I shall open is that which may be 
termed the argument from Classification. 
It is a matter of observable fact that different forms 
of plants and animals present among themselves more 
or less pronounced resemblances. From the earliest 
times, therefore, it has been the aim of philosophical 
naturalists to classify plants and animals in accord- 
ance with these resemblances. Of course the earliest 
attempts at such classification were extremely crude. 
The oldest of these attempts with which we are ac- 
quainted—namely, that which is presented in the books 
of Genesis and Leviticus—arranges the whole vegetable 
kingdom in three simple divisions of Grass, Herbs, and 
Trees; while the animal kingdom is arranged with 
almost equal simplicity with reference, first to habitats 
in water, earth, or air, and next as to modes of pro- 
gression. These, of course, were what may be termed 
common-sense classifications, having reference merely 
to external appearances and habits of life. But when 
Aristotle laboriously investigated the comparative 
anatomy of animals, he could not fail to perceive that 
their entire structures had to be taken into account in 
