nd
shuts at intervals, like the jaws of the pedicellariae of the echinus, and
there is altogether, in general principle, a remarkable similarity between
the structures. Yet the echinus can have, at the best, none but the most
distant genetic relationship with the Polyzoa. We have here again therefore
complex and similar organs of diverse and independent origin.

[Illustration: BIRD'S-HEAD PROCESSES VERY GREATLY ENLARGED.]

In the highest class of animals (the Mammalia) we have almost always a
placental mode of reproduction, _i.e._ the blood of the foetus is placed in
nutritive relation with the blood of the mother by means of vascular
prominences. No trace of such a structure exists in any bird or in any
reptile, and yet it crops out again in certain sharks. There indeed it
might well be supposed to end, but, marvellous as it seems, it reappears in
very lowly creatures; namely, in certain of the ascidians, sometimes called
tunicaries or sea-squirts. [Page 82]

Now, if we were to concede that the ascidians were the common ancestors[61]
of both these sharks and of the higher mammals, we should be little, if
any, nearer to an explanation of the phenomenon by means of "Natural
Selection," for in the sharks in question the vascular prominences are
developed from one foetal structure (the umbilical vesicle), while in the
higher mammals they are developed from quite another part, viz. the
allantois.

[Illustration: Upper Figure--ANTECHINUS MINUTISSIMUS (_implacental_).

Lower Figure--MUS DELICATULUS (_placental_).]

So great, however, is the number of similar, but apparently independent,
structures, that we suffer from a perfect _embarras de richesses_. Thus,
for example, we have the convoluted windpipe of the sloth, reminding us{83}
of the condition of the windpipe in birds; and in another mammal, allied to
the sloth, namely the great ant-eater (Myrmecophaga), we have again an
ornithic character in its horny gizzard-like stomach. In man and the
highest apes the caecum has a vermiform appendix, as it has also in the
wombat!

Also the similar forms presented by the crowns of the teeth in some seals,
in certain sharks, and in some extinct Cetacea may be referred to; as also
the similarity of the beak in birds, some reptiles, in the tadpole, and
cuttle-fishes. As to entire external form, may be adduced the wonderful
similarity between a true mouse (_Mus delicatulus_) and a small marsupial,
pointed out by Mr. Andrew Murray