ON INSTINCT. 
343 
first state, and subsequently becomes herbivorous. Another, on 
the contrary, is first nourished on the debris of vegetables^ and 
afterwards cannot live except on the fluid or solid parts of animal 
bodies. Some, during a period of their existence, can absorb or 
suck their aliment in a liquid form, and, consequently, without 
mastication; while, under other circumstances, the parts of their 
mouth having changed form^ they will attack nothing but solids. 
Here we have striking examples of the change of instinct, 
according to the change of the organization ; and in the course of 
this Lecture I shall produce many facts to prove, that the 
peculiar instincts of animals depend on their peculiar organi¬ 
zation. This is one of the principal positions which I hope to 
establish. 
Comparative anatomy affords the strongest and most numer¬ 
ous proofs of the dependence of function on structure. Every 
variation in the construction of an organ is accompanied with a 
corresponding modification of function ; and—to cut the matter 
short by an example or two—is the vital economy of an insect the 
same as that of a fish ? or does that of either resemble the phy¬ 
siology of a quadruped ? Do the very different teeth, jaws, 
muscles, stomach, and intestines of a cow and a lion perform 
the same offices? The visible fabric of the brain differs most 
widely in quadrupeds, birds, fishes, and insects. Is there not 
an equal difference in their intellectual phenomena, appetites, 
and instincts? 
In commencing a survey of organized nature, I hardly know 
where to set out, so minutely regular is the connecting chain, so 
close is the analogy that links all, and so beautiful is the har¬ 
mony that prevails throughout. Let us imagine such a chain, 
and, beginning with the most perfect and continued downwards, 
we shall find a tolerably regular gradation from complicated to 
simple through the whole series. At one end is man, at the 
other an animated microscopic drop of fluid : numberless grada¬ 
tions are placed between these, so that, though the two ends of 
the chain are immeasurably remote, there is close approxima¬ 
tion betw'een any two links. 
It would certainly be ridiculous to attempt to speculate on the 
feelings of the animalculae, whose world is a drop of fluid, and 
whose fleeting existence, chequered, perhaps, by various trans¬ 
formations, is destined to run its course in a few hours; yet 
nothing is more certain than this, that a link of sensitive exist¬ 
ence may be traced from the animalcukcE to the philosopher. I 
have scarcely occasion to remind you, that the number and kind 
of the intellectual phenomena discovered in different animals 
correspond closely to the degree of development of the brain. 
