Theory of Recapitulation 175 
generalisation known as the Law of v. Baer. The law asserts that 
embryos of different species of animals of the same group are more 
alike than the adults and that, the younger the embryo, the greater 
are the resemblances, If this law could be established it would 
undoubtedly be a strong argument in favour of the “recapitu- 
lation” explanation of the facts of embryology. But its truth has 
been seriously disputed. If it were true we should expect to find 
that the embryos of closely similar species would be indistinguishable 
from one another, but this is notoriously not the case. It is more 
difficult to meet the assertion when it is made in the form given 
above, for here we are dealing with matters of opinion. For instance, 
no one would deny that the embryo of a dogfish is different from the 
embryo of a rabbit, but there is room for difference of opinion when 
it is asserted that the difference is less than the difference between an 
adult dogfish and an adult rabbit. It would be perfectly true to say 
that the differences between the embryos concern other organs more 
than do the differences between the adults, but who is prepared to 
affirm that the presence of a cephalic coelom and of cranial segments, 
of external gills, of six gill slits, of the kidney tubes opening into the 
muscle-plate coelom, of an enormous yolk-sac, of a neurenteric canal, 
and the absence of any trace of an amnion, of an allantois and of a 
primitive streak are not morphological facts of as high an import as 
those implied by the differences between the adults? The generalisa- 
tion undoubtedly had its origin in the fact that there is what may be 
called a family resemblance between embryos and larvae, but this 
resemblance, which is by no means exact, is largely superficial and 
does not extend to anatomical detail. 
It is useless to say, as Weismann has stated}, that “it cannot 
be disputed that the rudiments [vestiges his translator means] of 
gill-arches and gill-clefts, which are peculiar to one stage of human 
ontogeny, give us every ground for concluding that we possessed fish- 
like ancestors.” The question at issue is: did the pharyngeal arches 
and clefts of mammalian embryos ever discharge a branchial function 
in an adult ancestor of the mammalia? We cannot therefore, without 
begging the question at issue in the grossest manner, apply to them 
the terms “gill-arches” and “gill-clefts.” That they are homologous 
with the “gill-arches” and “gill-clefts” of fishes is true; but there is 
no evidence to show that they ever discharged a branchial function. 
Until such evidence is forthcoming, it is beside the point to say that 
it “cannot be disputed” that they are evidence of a piscine ancestry. 
It must, therefore, be admitted that one outcome of the progress 
of embryological and palaeontological research for the last 50 years 
1 The Evolution Theory, by A. Weismann, English Translation, Vol. u. p. 176, 
London, 1904. 
