176 
ANIMAL PEDIGREES. 
Aug., 1891 . 
step. The gain here is increased distinctness and increased 
brightness of the image, for the lens will focus the rays of 
light more sharply on the retina, and will allow a greater 
quantity of light, a larger pencil of rays from each part of 
the object, to reach the corresponding part of the retina. The 
eye is now in the condition in which it remains throughout 
life in the snail and other gastropods. Finally the formation 
of the folds of skin known as iris and eyelids provides for the 
better protection of the eye, and is a clear advance on the 
somewhat clumsy method of withdrawal seen in the snail. 
It is not always possible to point out so clearly as in the 
above instance the particular advantage gained at each step, 
even when a complete developmental series is known to us ; 
but in such cases, as for instance in Orbitolites, our difficul¬ 
ties may be largely ascribed to ignorance of the particular 
conditions that confer advantage in the struggle for existence 
in the case of the forms we are dealing with. 
That ontogeny really is a repetition of phylogeny must, 
I think, be admitted, in spite of the numerous and various 
ways in which the ancestral history may be distorted during 
actual development. 
Before leaving the subject, it is worth while inquiring 
whether any explanation can be found of recapitulation. A 
complete answer can certainly not be given at present, but 
a partial one may, perhaps, be obtained. 
Darwin himself suggested that the clue might be found 
in the consideration that at whatever age a variation first 
appears in the parent, it tends to reappear at a correspond¬ 
ing age in the offspring; but this must be regarded rather as 
a statement of the fundamental fact of embryology than as 
an explanation of it. 
It is probably safe to assume that animals would not 
recapitulate unless they were compelled to do so : that there 
must be some constraining influence at work, forcing them to 
repeat more or less closely the ancestral stages. It is 
impossible, for instance, to conceive what advantage it can 
be to a reptilian or mammalian embryo to develop gill-clefts 
which are never used, and which disappear at a slightly 
later stage ; or how it can benefit a whale, that in its 
embrvonic condition it should possess teeth which never cut 
•/ i 
the gum, and which are lost before birth. 
Moreover, the history of development in different animals 
or groups of animals offers to us, as we have seen, a series 
of ingenious, determined, varied, but more or less unsuccess¬ 
ful efforts to escape from the necessity of recapitulating, and 
to substitute for the ancestral process jx, more direct method. 
