stations, here and there arrests its own development, and
thus determines at each point of interruption, by the very state it has
reached, the distinctive characters of the phyla, the classes, families,
genera, and species" (p. 833).[323]

To settle the dispute pending between two of its most illustrious
members, the Academy proposed in 1853, as the subject of one of its
prizes, "the positive determination of the resemblances and differences
in the comparative development of Vertebrates and Invertebrates." A
memoir was presented the next year by Lereboullet[324] which met with the
approval of the Academy in so far as its statements of fact were
concerned, but seemed to them to require amplification in its
theoretical part. But even in this memoir Lereboullet was able to show
that the balance of evidence was greatly in favour of Milne-Edwards'
views, and his general conclusions in 1854 were that "in the presence of
such fundamental differences, one is obliged to give up the idea of one
single plan in the formation of animals; while, on the contrary, the
existence of diverse plans or types is clearly demonstrated by all the
facts" (p. 79). To fulfil the Academy's requirements, Lereboullet
continued his work, and in 1861-63 he published a series of elaborate
monographs[325] on the embryology of the trout, the lizard and the
pond-snail _Lymnaea_, and rounded off his work with a full discussion[326]
of the theoretical questions involved. In this considered and
authoritative judgment he completely disposed of Serres' theories of the
unity of plan and the unity of genetic formation. Except in the very
earliest stages of oogenesis there is no real similarity between the
development of a Zoophyte, a Mollusc, an Articulate and a Vertebrate,
but each is stamped from the beginning with the characteristics of its
type. The lower animals are not, and cannot possibly be the permanent
embryos of the higher animals. "The results which I have obtained," he
writes, "are diametrically opposed to the theory of the zoological
series constituted by stages of increasing perfection, a theory which
tries to demonstrate in the embryonic phases of the higher animals a
repetition of the forms which characterise the lower animals, and which
has led to the assertion that the latter are permanent embryos of the
former. The embryo of a Vertebrate shows the vertebrate type from the
very beginning, and retains this type throughout the whole course of its