2 FOOT-NOTES TO EVOLUTION. 
published by Linneus in 1758. This book treats of all 
the species of animals known a century and a half ago. 
In its eight hundred and twenty-three 
pages some four thousand different kinds 
of animals are named and described. 
But for every one of these enumérated by Linnzus, more 
than two hundred kinds are known to the modern natu- 
ralist, and the number of species still unknown doubt- 
less exceeds that of those already recorded. Every year 
since 1864 there has been published in London a plump 
octavo volume known as the Zodélogical Record. Each 
of these volumes, larger than the whole Systema Natura, 
contains the names of animals new to science added to 
our list during the year of which it treats. And in the 
record of each year we find the names of about three 
times as many animals as are mentioned in the Systema 
Nature. Yet the field shows no signs of exhaustion. 
As these volumes stand on the shelf together it is easy 
to see that the later volumes are the thickest, and that 
the record for the present year is the largest of all. 
Moreover, what is true of the increase of knowledge in 
systematic zodlogy is even more marked in the case of 
botany. Such, then, is the variety of life on the globe 
—a variety of which Linnzus and his successors had 
never dared to dream. 
And yet, great as this variety is, there are, after all, 
only a few types of structure among all animals and 
plants, some three or four or eight or 
ten general modes of development, and 
all the rest are modifications from 
these few types. It is, moreover, true that all living 
forms are but series of modifications and extensions of 
one single plan of structure. All have the same frame- 
work of cells, and in each cell we find the same ultimate 
substance—the mysterious semi-fluid network of proto- 
The number of 
species. 
The unity of 
type. 
