358 NATURAL HISTORY. 
senses and in that of voluntary motion, and also in the 
nervous system, by which these two kinds of apparatus 
are connected with the mind.* In the lower orders of 
animals the senses are very imperfectly developed, and 
in some most of the senses are absent. Thus, in the Hy- 
dra (§ 599) and in the Actiniz (§ 576) there is no evi- 
dence of the existence of but one sense, that of touch. 
The Actiniz are, indeed, sensibly affected by light, but 
this does not prove that they see. As we go upward in 
the scale we find the apparatus of the senses generally 
more and more developed. Taking all of them into view, 
the senses are best developed in man, though some of 
them, for special purposes, have a higher capacity in cer- 
tain animals than in him. Some may have a more acute 
smell, as the dog, or see farther, as the eagle; but no an- 
imal has ail the senses in such perfection as man. The 
same can be said of the muscular apparatus. The vari- 
ety of muscular action is greatest of all in man, while in 
some animals there are special muscular endowments for 
special purposes above any thing of the kind to be found 
in him. The gradation in the nervous system is still 
more definitely marked. In man it has its fullest devel- 
opment ; and, as we go down in the scale, we at length 
come to animals that have no distinct brain, and finally 
to those in which, as the hydra, no trace of any thing 
like a nerve can be found. In these last nervous matter 
is presumed to exist because actions are performed which, 
in animals of a more defined organization, are known to 
be dependent upon nervous agency. 
624. Amid all the variations of structure to suit the 
different wants and capabilities of animals, the Creator 
has adopted certain general plans, so that order prevails 
throughout all the extreme variety of the animal king- 
dom. We can see this whether we take into view large 
* For the relations of the senses, the muscles, and the nervous sys- 
tem, I refer you to the chapter on the Nervous System in my ‘‘ First 
Book in Physiology.” 
