e never come upon ears again in any lower class.

"Not so with the organ of sight: we see this organ disappear, reappear,
and disappear again with the possibility or impossibility of using eyes
on the part of the creature itself.[374]

"The great development of mantle in the acephalous molluscs has rendered
eyes, and even a head, entirely useless to them. These organs, though
belonging to the type of the organism, and by rights included in it,
have had to disappear and become annihilated owing to continued default
of use.

. . . . . .

"Many insects which, by the analogy of their order and even genus,
should have wings, have nevertheless lost them more or less completely
through disuse. A number of coleoptera, orthoptera, hymenoptera, and
hemiptera give us examples, the habits of these animals never leading
them to use their wings."[375]

* * * * *

I will here bring this present volume to a conclusion, hoping, however,
to return to the same subject shortly, but to that part of it which
bears upon longevity and the phenomena of old age. In 'Life and Habit' I
pointed out that if differentiations of structure and instinct are
considered as due to the different desires under different circumstances
of an organism, which must be regarded as a single creature, though its
development has extended over millions of years, and which is guided
mainly by habit and memory until some disturbing cause compels
invention--then the longevity of each generation or stage of this
organism should depend upon the lateness of the average age of
reproduction in each generation; so that an organism (using the word in
its usual signification) which did not upon the average begin to
reproduce itself till it was twenty, should be longer lived than one
that on the average begins to reproduce itself at a year old. I also
maintained that the phenomena of old age should be referred to failure
of memory on the part of the organism, which in the embryonic stages,
infancy, youth, and early manhood, leans upon the memory of what it did
when it was in the persons of its ancestors; in middle life, carries its
action onward by means of the impetus, already received, and by the
force of habit; and in old age becomes puzzled, having no experience of
any past existence at seventy-five, we will say, to guide it, and
therefore forgetting itself more and more completely till it dies. I
hope to extend this, and to bring forward argu