INTRODUCTION 9 
brates, but also for invertebrates, including even totally extinct types. In 
ammonites, for instance, the primary or innermost whorls always differ from 
the outer in their greater simplicity of suture, and in their lesser ornamenta- 
tion. Very often a similarity is observable with geologically older forms ; 
and it is a well-known fact that all ammonites pass through early stages which 
resemble, at least so far as chambering of the shell is concerned, Palaeozoic 
goniatites. A comparison of the inner whorls of an ammonite with its corre- 
sponding goniatitic form, or with older ammonites, seldom fails to reveal 
ties of kinship not otherwise discernible. Beecher has shown that nearly 
every stage in the growth of arm-supports in recent brachiopods corresponds 
to some fossil genus; and further, that the chronological succession of the 
latter is to a certain degree identical with the successive ontogenetic stages 
of recent forms. 
The relation of so-called rudimentary organs occurring in recent forms to 
those of the allied predecessors of the latter is of particular significance. 
By rudimentary organs are meant certain structures (as, for example, 
limbs, parts of aales. organs of sense, respiration, digestion, reproduc- 
tion, ete.), which are still indicated by dwarfed remains, but whose 
physiological functions, and hence their utility to the organism, have wholly 
disappeared. Rudimentary organs are, as a rule, either normally de- 
veloped in an embryonic stage, or at least more strongly than in the adult 
individual, owing to a process of degeneration, or retrogressive development. 
The fossil progenitors of forms possessing rudimentary organs are almost 
always characterised by a full development of the respective parts. The 
lateral metacarpals and metatarsals in the horse and most ruminants, for 
example, are indicated only by rudimentary side-splints ; but in an embryonic 
stage they are much more strongly developed, and in related fossil forms they 
occur as normal bones, carrying toes like the other metapodals, and serving 
for locomotion and support. The wrist and metacarpal bones in birds have 
also suffered degeneration, as is evident from a comparison with embryoes 
and with older forms (Archaeopteryx), which exhibit a much higher develop- 
ment. In like manner, the teeth of birds have also become degenerated. In 
only a few forms (parrots) are indications of dental ridges discernible 
during embryonic stages; but in all known Mesozoic birds the teeth are 
well developed and remain functional throughout life. Similarly, teeth are 
developed during embryonic stages in the baleen whale, but subsequently 
become atrophied ; while in the older fossil Cetacea teeth are always present. 
Other instances of this nature are to be met with in great profusion, both 
among vertebrates and invertebrates. 
The biogenetic law is, however, not infrequently obscured, for the reason 
that two closely related forms may not develop in exactly the same manner ; 
embryoes of the one type may be affected by peculiar accelerating impulses 
which are not shared by those of the other, and in consequence the first may 
pass through certain stages very rapidly, or may even omit them altogether. 
In this way the historical or palingenetic record contained in the growth of 
every individual may be to a large extent suppressed or rendered unintelligible ; 
and this phenomenon of inexact parallelism (coenogenesis) is especially common 
in highly differentiated types, where the embryo passes through a multitude 
of phases. 
Palaeontology and Phylogeny.—While conceding that by means of 
