14 INTRODUCTION. 
As regards animals, all alike, whatever their structure may 
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 concerned, 
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 itself as per- 
fectly, 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 function, and it 
is attained as fully and perfectly by the sponge as it is by man 
-himself. The same holds good of reproduction. Whilst 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 re- 
gards the functions of relation, on the other hand, we have 
every possible grade of perfection exhibited as we ascend from 
the lowest members of the animal kingdom to the highest. So 
numerous, in fact, are the changes in these functions, 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 bead between the 
higher and lower animals. It may reasonably be doubted 
whether in this respect a horse or a dog has anything in com- 
mon with a sponge. 
_ Instead of giving here a general sketch of each of the great phy- 
siological functions as a whole, it may be as well to accompany 
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 perfection in 
passing from the bottom to the summit of the animal series. 
HOMOLOGY AND ANALOGY.—In connection with the morpho- 
