Chap. I.] INTRODUCTORY. 



Descriptive Biology is baseless speculation. Both must grow 

 together and minister to each other's wants. 



It only remains, in this introductory chapter, to indicate the 

 aim and scope of this volume. 



The number of animals each a centre of so many biological 

 problems is well-nigh countless. So is the number of houses 

 in England. And yet, on entering the house of an acquaintance 

 whom we have never before visited, we know at once what we 

 shall find. The hall and passages, the dining-room, drawing- 

 room, library; kitchen and offices below; bedrooms above we 

 already more than half know them all. And why ? Because 

 we already know the type of house that belongs to a certain 

 grade of society. If we visit a lord or a labourer, our expecta- 

 tions are different. We know that some men have to live in 

 hovels, in which one room has to suffice for all the needs of the 

 family. We know that at the other end of the scale of social 

 life there is a separate room for every function of that more 

 complex life. So is it also with animals. There is a lowly type 

 of animal, where a single cell constitutes the whole house, and 

 all the functions of life have to be performed in and through 

 that single cell. There are others composed of many cells, in 

 which differentiation of structure and specialisation of function 

 have been carried far. But a special type of structure is 

 characteristic of each grade of animal life, just as a special type 

 of house characterises each grade of social life. And if we know 

 the type which marks that special grade, we more than half 

 know what will be the state of matters with regard to differen- 

 tiation of structure and specialisation of function in any indi- 

 vidual case. If we know, for example, the tvjpical structure of a 

 mammal, a bird, and an amphibian, we know pretty well what 

 to expect in any other mammal, bird, and amphibian, and are 

 some way on our road towards a knowledge of the structure of 

 a fish or a reptile. And so with regard to other grades of 

 animal life. 



It is the object of this volume to assist the student in acquir- 

 ing such a knowledge of some of the more important facts of the 

 morphology and physiology of certain typical animals as may form 



