RELATIVES OF THE OYSTER. 195 



professo, to reflect, that, as commonly employed, the term 

 has a double sense, and denotes two very different orders 

 of relations. When we call a group of animals, or of 

 plants, a species, we may imply thereby, either that all 

 these animals or plants have some common peculiarity of 

 form or structure, or we may mean that they possess some 

 common functional character. That part of biological 

 science which deals with form and structure is called Mor- 

 phology that which concerns itself with function, Physi- 

 ology (d] so that we may conveniently speak of these two 

 senses, or aspects, of " species ' -the one as morphological, 

 the other as physiological. 



(d) " . . . . It remains that I should put before you what I 

 understand to be the third phase of geological speculation namely, 

 EVOLUTIONISM. 



" I shall not make \vhat I have to say on this head clear, unless I 

 diverge, or seem to diverge, for a while, from the path of my discourse, 

 so far as to explain what I take to be the scope of geology itself. I 

 conceive geology to be the history of the earth, in precisely the same 

 sense as biology is the history of living beings ; and I trust you will 

 not think that I am overpowered by the influence of a dominant pur- 

 suit if I say that I trace a close analogy between these two histories. 



" If I study a living being, under what heads does the knowledge 

 I obtain fall ? I can learn its structure, or what we call its ANATOMY, 

 and its DEVELOPMENT, or the series of changes which it passes through 

 to acquire its complete structure. Then I find that the living being has 

 certain powers resulting from its own activities, and the interaction of 

 these with the activities of other things the knowledge of which is 

 PHYSIOLOGY. Beyond this the living being has a position in space 

 and time, which is its DISTRIBUTION. All these form the body of 

 ascertainable facts which constitute the status quo of the living creature. 

 But these facts have their causes ; and the ascertainment of these 

 causes is the doctrine of ^ETIOLOGY. 



"If we consider what is knowable about the earth, we shall find 

 that such earth-knowledge if I may so translate the word geology 

 falls into the same categories. 



G 2 



