THEORIES OF EVOLUTION. 
An Historical Outline. 
AGNES ROBERTSON, D.Sc. 
I.—INTRODUCTION. 
ALL the animals and plants to-day inhabiting the world can be 
classified into small groups known as ‘species.’ This is recog- 
nised even by those who are not naturalists. We call an animal 
having a certain set of characters a horse, and a plant having a 
certain set of characters a dandelion, using these names without 
hesitation, although the individuals which we identify in this 
way differ considerably among themselves. We have in our 
minds a conception of a typical horse and of a typical dandelion, 
and an animal or a plant conforming to one of these types more 
closely than to any other, we regard as belonging to the horse 
or dandelion ‘species.’ The world, then, is peopled with a huge 
number of species which, on the whole, are sharply distinguish- 
able, and further, these species are not altogether isolated, but 
can be classified into groups by means of the characters which 
they possess in common. For instance the Field Buttercup, 
the Lesser Celandine, and the Water Crowfoot, though no one 
would ever confuse them, yet resemble one another closely enough 
to be placed in the same group of similar species, or, as we say, in 
the same ‘genus.’ Again, it is found that these genera can them- 
selves be arranged in larger groups called ‘orders.’ The 
species belonging to a genus resemble one another more closely 
than they do those of any other genus, and in the same way the 
genera belonging to the same order show more resemblance to 
one another than to the genera belonging to any other order. 
Similar orders can be grouped into larger classificatory units, and 
so on. 
So far, we have been speaking of the organic world 
descriptively; the fact that organised beings naturally fall 
into a classification of the kind just outlined would be allowed 
by everyone, and is not a matter of theory, but merely of 
observation. But when we turn to the question of ow this 
state of things has arisen, we plunge at once into the region of 
hypothesis. There are two main contrasting views ; those of 
Special Creation and of Descent with Modification. According 
to the first view, the species of the animal and plant world were 
1907 May 1. 
