SECTION I. 

 GENERAL PRELIMINARY CONSIDERATIONS. 



CHAPTER I. 



THE PHYSIOLOGY OP OKGANISMS. 



The general direction of zoology is, as we have seen in the 

 introduction, determined by two branches of science — Morpho- 

 logy and Physiology. Although both make it their task to 

 learn to understand the phenomena presented to us by the 

 animal kingdom, they are so widely dififerent, both as to their 

 details and as to the paths they have struck out for solving the 

 problems, that we are fully justified in keeping them separate 

 as two independent branches of science. 



The problem for Morphology is to discover those affinities 

 of relationship in animals which actually exist, and to found on 

 them a natural system of the animal kingdom. It attains this 

 end, or endeavours to attain it, by investigating morphological 

 differences, as well as those similarities which indicate a true 

 affinity, by means of the comparative method — comparative 

 anatomy and embryology. Physiology, on the other hand, does 

 not seek to establish those affinities, but, on the contrary, to 

 investigate those universal conditions of existence and those 

 functions of living organisms which may elucidate from the 

 point of view of the laws of causation, among other things, the 

 natural system arrived at from morphology. Morphology, indeed, 

 only establishes the relations of affinity between individual 



