28 ELEMENTS OF BIOLOGY. 



differ from any another ; and they may be considered re- 

 spectively under the heads of " Specialisation of Functions" 

 and " Morphological Type." 



a. Specialisation of Functions. All animals alike, what- 

 ever their structure may be, perform the three great phy- 

 siological functions ; that is to say, they all nourish them- 

 selves, reproduce their like, and have certain relations with 

 the external world. They differ from one another physio- 

 logically in the manner in which these functions are per- 

 formed. 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 reproduction, as far as their 

 results are concerned, are essentially the same in all ani- 

 mals. In the manner, however, in which the same results 

 are brought about, great differences are observable in dif- 

 ferent 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. It 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 second- 

 ary functions which characterise the process in the higher 

 animals. It is the same, too, with the functions of repro- 

 duction 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. 



In the simpler Protozoa, such as the Proteus animalcule 

 or Amoeba (fig. 4), it can hardly be said that there are any 



