468 TRANSACTIONS OF SECTION C. 
replaced by the cats, sabre-toothed tigers, and dogs, which came down south 
from North America over the newly emerged isthmus of Panama at the close 
of the Pliocene period. At least, the remains of these old carnivores and 
their immigrant successors have never yet been found associated in any 
geological formation. 
These various considerations lead me to think that there is also deep 
significance in the tendency towards fixity in the number and regularity (or 
symmetry) in the arrangement of their multiple parts which we frequently 
observe in groups of animals as we trace them from their origin to their 
prime. It is well known that in certain of the highest and latest types 
of bony fishes the vertebre and fin-rays are reduced to a fixed and practically 
invariable number for each family or genus, whereas there is no such fixity 
in the lower and earlier groups. In the earliest known Pycnodont fishes from 
the Lower Lias (Mesodon) the grinding teeth form an irregular cluster, 
while in most of the higher and later genera they are arranged in definite 
regular rows in a symmetrical manner. Many of the lower backboned 
animals have teeth with several cusps, and in some genera the number of 
teeth seems to be constant; but in the geological history of the successive 
classes the tooth-cusps never became fixed individual entities readily trace- 
able throughout whole groups, until the highest or mammalian grade had 
been attained. Moreover, it is only in the same latest grade or class that 
the teeth themselves can be treated as definite units, always the same in 
number (forty-four), except when modified by degeneration or special adapta- 
tion. In the earlier and lower land animals the number of vertebre in the 
neck depends on the extent of this part, whereas in the mammal it is almost 
invariably seven, whatever the total length may be. Curiously constant, 
too, in the modern even-toed hoofed mammals is the number of nineteen 
vertebrae between the neck and the sacrum. 
I am therefore still inclined to believe that the comparison of vital pro- 
cesses with certain purely physical phenomena is not altogether fanciful. 
Changes towards advancement and fixity which are so determinate in 
direction, and changes towards extinction which are so continually repeated, 
seem to denote some inherent property in living things which is as definite 
as that of crystallisation in inorganic substances. The regular course of 
these changes is merely hindered and modified by a succession of checks from 
the environment and Natural Selection. Each separate chain of life, indeed, 
bears a striking resemblance to a crystal of some inorganic substance which 
has been disturbed by impurities during its growth, and has thus been 
fashioned with unequal faces, or even turned partly into a mere concretion. 
In the case of a crystal the inherent forces act solely on molecules of the 
crystalline substance itself, collecting them and striving, even in a disturbing 
environment, to arrange them in a fixed geometrical shape. In the case of 
a chain of life (or organic phylum) we may regard each successive animal as 
a temporary excrescence of colloid substance round the equally colloid germ- 
plasm which persists continuously from generation to generation. The 
inherent forces of this germ-plasm, therefore, act upon a consecutive series 
of excrescences (or animal bodies) struggling, not for geometrically arranged 
boundaries, but towards various other symmetries, and a fixity in number 
of multiple parts. When the extreme has been reached, activities cease, and 
sooner or later the race is dead. 
Such are some of the most important general results to which the study 
of fossils has led during recent years ; and they are conclusions which every 
new discovery appears to make more certain. When we turn to details, 
however, it must be admitted that modern systematic researches are con- 
tinually complicating rather than simplifying the problems we have to solve. 
Professor Charles Depéret has lately written with scant respect of some of 
the pioneers who were content with generalities, and based their conclusions 
on the geological succession of certain anatomical structures rather than on 
