14 INTRODUCTION. 



be, perform the three great physiological functions that is to 

 say, they all nourish themselves, reproduce their like, directly or 

 indirectly, and have certain relations with the external world. 

 When we come, however, to compare animals together physio- 

 logically, it is soon seen that the functions of relation stand in 

 quite a different position to that occupied by the functions of 

 nutrition and reproduction. As far as these last are con- 

 cerned, there can be no difference in the amount or perfection 

 of the function discharged by the organism. The simplest 

 and most degraded of animals say a sponge nourishes it- 

 self as perfectly, as far as the result to itself is concerned, as 

 does the highest of animals. Nutrition can do no more than 

 maintain the body of any animal in a healthy and vigorous 

 condition. This is the highest possible perfection of the func- 

 tion, and it is attained as fully and perfectly by the sponge as 

 it is by man himself. The same holds good of reproduction. 

 While the functions of nutrition and reproduction are thus, as 

 regards their essence and results, the same in all animals, it 

 must be remembered that there are enormous differences in 

 the manner in which the functions are discharged. The result 

 attained is in all cases the same, but it may be arrived at in 

 the most different ways and with the most different apparatus. 

 As regards the functions of relation, on the other hand, we 

 have every possible grade of perfection exhibited as we as- 

 cend from the lowest members of the animal kingdom to the 

 highest. So numerous, in fact, are the changes in these func- 

 tions, and so great the additions which are made in the higher 

 organisms, that it may be doubted if there exists any common 

 element by which a comparison can be drawn on this head be- 

 tween the higher and lower animals. It may reasonably be 

 doubted whether in this respect a horse or a dog has any 

 thing in common with a sponge. 



Instead of giving here a general sketch of each of the great 

 physiological functions as a whole, it may be as well to accom- 

 pany the morphological account of each primary division of 

 animals with a short account of the manner in which the vital 

 functions are carried out in the same. In this way a clearer 

 view will be obtained of the gradual rise in physiological per- 

 fection in passing from the bottom to the summit of the ani- 

 mal series. 



HOMOLOGY AND ANALOGY. In connection with the mor- 

 phological and physiological differences between animals, a 

 . short explanation may be given of the meaning of the terms 

 Homology and Analogy, which are in constant use in zoologi- 



