DIFFERENCES BETWEEN DIFFERENT ANIMALS. 19 



functions of correlation are called the animal functions, as 

 being more especially characteristic of, though not peculiar to, 

 animals. 



7. DIFFERENCES BETWEEN DIFFERENT ANIMALS. 



All the innumerable differences which subsist between dif- 

 ferent animals may be classed under two heads, corresponding 

 to the two aspects of every living being, morphological and 

 physiological. One animal differs from another either morpho- 

 logically ', in the fundamental points of its structure ; or physio- 

 logically, in the manner in which the vital functions of the 

 organism are discharged. These constitute the only modes in 1 

 which anyone animal can differ from any other; and they may \ 

 be considered respectively under the heads of Specialisation of 

 Function and Morphological type. 



a. Specialisation of Function. All animals alike, whatever 

 their structure may be, perform the three great physiological 

 functions ; that is to say, they all nourish themselves, repro- 

 duce their like, and have certain relations with the external 

 world. They differ from one another physiologically in the 

 manner in which these functions are performed. Indeed, it is 

 only in the functions of correlation that it is possible that there 

 should be any difference in the amount or perfection of the 

 function performed by the organism, since nutrition and repro- 

 duction, as far as their results are concerned, are essentially the 

 same in all animals. In the manner, however, in which the 

 same results are brought about, great differences are observable 

 in different animals. The nutrition of such a simple organism 

 as the Amoeba is, indeed, performed perfectly, as far as the 

 result to the animal itself is concerned as perfectly as in the 

 case of the highest animal but it is performed with the simplest 

 possible apparatus. Jt may, in fact, be said to be performed 

 without any special apparatus, since any part of the surface of 

 the body may be extemporised into a mouth, and there is no 

 differentiated alimentary cavity. And not only is the nutritive 

 apparatus of the simplest character, but the function itself is 

 equally simple, and is entirely divested of those complexities 

 and separations into secondary functions which characterise 

 the process in the higher animals. It is the same, too, with 

 the functions of reproduction and correlation; but .this point 

 will be more clearly brought out if we examine the method in 

 which one of the three primary functions is performed in two 

 or three examples. Nutrition, as the simplest of the functions, 

 will best answer the purpose. 



