MOLLUSKS 91 
the eye of the squids and cuttlefishes has shown them 
to be remarkably complex and in many respects to be 
constructed upon much the same plan as those of the 
vertebrates. As to the other senses not so much is known, 
but undoubtedly many species of cephalopods are possessed 
of a shrewdness and cunning not shared by any other 
invertebrates, save some of the insects and spiders, and are 
vastly more highly organized than their molluscan rela- 
tives. 
91. How species originate.—We have now examined a 
considerable portion of the animal kingdom, tracing its 
members from their simplest beginnings as single cells, 
through the formation of colonial types, and up through 
the sponges, celenterates, worms, and mollusks. It is im- 
portant once more to note that they all perform the func- 
tions concerned in nutrition and reproduction, and only 
these. The differences which exist are those of structure. 
The Hydra and the clam, for example, perform the same 
duties, but their bodily apparatus differs widely, and the 
completeness and perfection of the work varies accord- 
ingly. The more the work to be performed by an organ- 
ism is divided up among especially adapted organs, so that 
each of the latter has, as far as possible, only one thing to 
do, the higher is the organism. 
As stated earlier in the account, it is believed that the 
more complex animals arose from the simpler; that if we 
could trace the history of any of the great groups back 
toward their first beginnings, we would find them all to 
have originated from one ancestral form, that in turn owes 
its descent from yet simpler forms. 
Let us see something of how this has come about. We 
all know that vast numbers of young are born into this 
world which never come to maturity. It is said that if all 
the young of the codfish were to live their allotted time, 
it would be less than fifteen years before the sea would 
become literally packed with them. Numerous enemies, 
