DIFFERENCES BETWEEN DIFFERENT ORGANISMS. 33 



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 repro- 

 duction 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 pos- 

 sible 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 organ- 

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

 element by which a comparison can be drawn on this head 

 between the higher and lower animals. It may reasonably 

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

 anything in common with a sponge. 



b. Morphological Type. The first point in which one ani- 

 mal may differ from another is the degree to which the 

 principle of the physiological division of labour is carried. 

 The second point in which one animal may differ from 

 another is in its " morphological type ; " that is to say, in 

 the fundamental plan upon which it is constructed. By one 

 not specially acquainted with the subject it might be readily 

 imagined that each species or kind of animal was con- 

 structed upon a plan peculiar to itself and not shared by 

 any other. This, however, is far from being the case ; and 

 it is now universally recognised that all the varied species of 

 animals however great the apparent amount of diversity 

 amongst them may be arranged under no more than half- 

 a-dozen primary morphological types or plans of structure. 



c 



