44 
Gravitation as a Factor 
[January, 
very considerably surpassing in size their nearest represen- 
tatives still living, and that those orders which now com- 
prise our bulkiest animals, such as the proboscidea and 
ungulata, were at one time far richer in species than at the 
present day. 
The relative magnitudes of the various animal groups do 
not seem to have been greatly different from what we now 
find them. Early geologic epochs certainly display birds 
larger than any now living. But the largest of these birds 
did not equal, much less exceed in stature, the large mam- 
malian forms of those days. The Harpagornis was smaller 
than the Machairodon , just as the harpy eagle of our 
time is smaller than the tiger or the jaguar. The mam- 
moth and the mastodon exceeded in hulk the Dinornis 
and AEpiornis, just as the elephant surpasses the ostrich. 
The rocks have yielded up remains of inserts larger, per- 
haps, than inserts of the same, or of closely approximating 
orders existing in the present day. But we find no proof 
that the Annulosa , as a class, made any nearer approach to 
the average growth of the Vertsbrata than what we observe 
in living species. This fadt, we submit, powerfully supports 
the view that the relative as well as the positive sizes of 
organic beings are not solely dependent upon such circum- 
stances as the greater or less abundance of food, the compe- 
tition of other species, &c., but are governed by some 
deeper law. Evolution has no tabula rasa upon which to 
work. We do not, of course, ignore or question the fadt that 
the respective parts played by certain of the leading groups 
of the animal kingdom have not always been the same ; 
that there has, for instance, been an epoch in the earth’s 
existence which might be pre-eminently considered as the 
age of reptiles, when the development of the mammalia 
was only in its infancy. 
That in so many cases the largest, strongest, and best- 
armed species should disappear, whilst the smaller and the 
weaker have been perpetuated, is a curious case of the survival 
of the fittest. This superiority demands an explanation. 
If, as seems not improbable, the density of the atmosphere 
was greater in the early geological epochs than is now the 
case, we have a clue to the more luxuriant growth of the 
terrestrial and aerial species of those days. They would 
then be placed in conditions somewhat approaching those 
of the aquatic animals, and might accordingly attain a 
larger size. 
It appears, therefore, that although gravitation sets cer- 
