92 PROF. W. GARSTANG ON THE THEORY OF RECAPITULATION : 
preferential action in place of the limited range of reflex and automatic 
mechanisms of more primitive types. That " little twist of brain," which dis- 
tinguishes one philosopher from another, is not more striking in its effects 
than are those trifling touches to the structure of the heart which transformed 
the cold-blooded Reptile and Stegocephalan into the warm-blooded Bird and 
Mammal respectively. Yet these are changes which, liowever graduated 
through successive generations at the outset, were not of a chai'acter to have 
been completed, or even initiated, in any adult stage of ontogeny. They must 
have been first manifested as a series of embryonic mutations, subjected 
continuously to selective tests of their relative physiological efficiency. Age 
bears the buffets of the world, but youth regenerates it, 
14. It is, however, the palasontologists who are the real defenders of the 
Biogenetic stronghold. With them the Law is a faith that inspires to deeds, 
while to the embryologist it is merely a text for disputation. The difference 
is striking and worth defining. When the embryologist sets up his larval 
images and worships them as prse-Cambrian ancestors, the real ancestors 
cannot be produced to demonstrate his folly. But the palESontologist's aim 
is to trace lineages directly, and he is not satisfied until he has produced his 
ancestors, or at least the most substantial remnants of them. I confess that 
I have been tempted many times before to-day to attack a theory which has 
led so many of us into blind alleys, but always Hyatt's Ammonites recurred 
to present an unanswered, and seemingly unanswerable case for Haeckelian 
recapitulation. A priori it seems absurd that senile characters should be the 
beginnings of a line of evolution (Hyatt, 1897, p. 221 &c.)i but the for- 
midable array of evidence, the wide range of unfamiliar material to be studied, 
and, not least, our ignorance of the habits and conditions of life of this type 
of Mollusk, have all conspired to render these Ammonites to me a real obstacle. 
The following case, however, has recently impressed me with its remarkable 
analogies, and justifies me, I hope, in presenting a general argument without 
directly tackling the Ammonite problem itself, at any rate for the present. 
The curious Prosobranch Gastropod Lamellaria, which mimics and devours 
Compound Ascidians, produces veliger larva? of a unique tyjie known as 
Echinospira (Krohn, 1853, 1857). The hyaline shell first produced is dilated 
so that it is far larger than its occupiant — resembling in this respect the 
gelatinous house of an Appendicularian. It is coiled like the shell of an 
Ammonite, being in some species discoidal, with perfect symmetry, in others 
spiral, and in the related Onchidiopsis more simple and sac-like (Bergh, 1887). 
The larva can withdraw himself completely, or, with his mantle-edge clasping 
the mouth of the shell, he can protrude a large 4- or 6-lobed velum, and 
swim about with it on his excursions with wonderful grace and ease. The 
mouth of the shell is regularly extended at its margin, the successive 
additions being marked by transverse lines of growth and generally by one 
or two pairs of longitudinal (spiral) rows of tubercles or spines as well. 
