4 THE DARWINIAN THEORY 



zoology, though for a time less perfectly than botany. 

 For the first time it became possible to think or 

 speak of the animal or vegetable kingdom com- 

 prehensively, and when a name was used, to know 

 precisely what it signified. This greater exactness 

 at once brought men face to face with the problem — 

 What is a species ? What do we mean by species 

 of animals or plants ? 



I will illustrate this by examples of what Linnaeus 

 meant by species. For instance, the jackdaw, the 

 raven, the rook, and the crow, are all species of the 

 genus Corvus. These birds are clearly more like 

 one another than either of them is like a starling 

 or an eagle; in other words, there is a certain 

 resemblance or affinity between them. Yet, 

 they always differ in certain slight peculiarities of 

 structure, form, and habits ; also, they always 

 produce their own kind, and do not interbreed — at 

 any rate, as a rule. The jackdaw (Corvus monedula) 

 is grey at the sides of its neck, and builds its nest in 

 holes or cavities in rocks, churches, chimneys, and 

 uninhabited houses. It feeds chiefly on insects, and 

 is much the smallest of the four birds. The raven 

 (Corvus corax) is the largest of the four, and is black 

 all over. It makes bulky nests on crags or in trees. 

 The rook (Corvus frugilegus) is a trifle smaller than 

 the crow, and lives in noisy flocks. It is black, with 

 a grey forehead and throat. Its nests are found in 

 small trees, often near human habitations. The 

 crow (Corvus corone) is smaller than the raven, and is 

 black, tinged with green on the neck and throat, and 

 purple on the back. So also the lion, tiger, leopard, 



