INTRODUCTORY 5 



labour, it by no means exhausts the matter. A 

 further investigation may lead him to find out that 

 his new discovery, though now so rare, was at one 

 period of the world's history a much more common 

 animal. There may be many fossil examples of a 

 similar creature. He must therefore describe its dis- 

 tribution in Time, or its Palceontology, as well as its 

 distribution in present space. The past and present 

 distribution of the same forms of life present some 

 most interesting problems, and the two do not by any 

 means necessarily coincide. Thus he has completed a 

 fourth aspect. But then there is another. What is it 

 that makes this animal different from all others, and 

 justifies its being regarded as a new form to science ? 

 What characters has it which are peculiar to itself ? 

 Some of its structures are common to many animals, 

 but some there are which no other animal possesses, 

 and on account of these the discovery is recognised as 

 a new species. This is the aspect of S;pecific characters. 

 Then there is an aspect which is really a combination 

 of the morphological and the physiological, but which 

 is such a large subject that it is regarded as a separate 

 aspect — namely, the study of the various changes in 

 form and function through which an organism passes 

 before it reaches the stage of maturity. This is 

 the embryological aspect, or the study of Emhryology. 

 Lastly, there is the philosophical aspect of animal life, 

 that which studies the connection of one kind of 

 animal with another, their relationships, their origin. 



