ry condition
of the development of the vertebral column, and so on. "Looked at from
this standpoint it cannot surprise us that in all animal phyla the
earliest embryonic processes take place in similar fashion, so that we
observe the occurrence both in Vertebrates and Invertebrates of a
segmentation-process, a morula-stage, a blastula and a gastrula. If now
these developmental processes do not depend on chance, but, on the
contrary, are rooted in the nature of the animal cell itself, we have no
reason for inferring from the recurrence of a similar
segmentation-process, morula, blastula, and gastrula in all classes of
the animal kingdom the common descent of all animals from one
blastula-like or gastrula-like ancestral form. We recognise rather in
the successive early stages of animal development only the manifestation
of special laws, by which the shaping of animal forms (as distinct from
plant forms) is brought about" (p. 178, 1906, b).

"The principal reason why certain stages recur in ontogeny with such
constancy and always in essentially the same manner is that they provide
under all circumstances the necessary pre-conditions through which alone
the later and higher stages of ontogeny can be realised. The unicellular
organism can by its very nature transform itself into a multicellular
organism only by the method of cell-division. Hence, in all Metazoa,
ontogeny must start with a segmentation-process, and a similar statement
could be made with regard to all the later stages" (p. 57, 1906, a).

Similarities in early development are therefore no evidence of common
descent, and in the same way the resemblances of adult animals, subsumed
under the concepts of homology and the unity of plan, are not
necessarily due to community of descent, but may also be brought about
by the similarity or identity of the laws which govern the evolution of
these animals. In the absence, therefore, of positive evidence as to the
actual lines of descent (to be obtained only from palaeontology),
homological resemblance cannot be taken as proof of blood relationship,
for homology is a wider concept than homogeny. The only valid definition
of homology is that adopted in pre-evolutionary days, when those organs
were considered homologous "which agree up to a certain point in
structure and composition, in position, arrangement, and relation to the
neighbouring organs, and accordingly possess identical functions and
uses in the organism" (p. 15