40 Gravitation as a Factor [January, 
contemplating, locomotion of this character would probably 
become common among the higher animals. The separation 
between the aerial and the terrestrial forms of life would be 
less marked than we now find. A true aerial life, moreover, 
would be much easier than at present, as the winged animal, 
of what kind soever, would require to exert much less force 
than at present in order to overcome the attraction of the 
earth. Hence the number, the variety, and probably the 
size of winged species would doubtless be much increased. 
We may safely assume that could gravitation be sufficiently 
intensified we should ultimately reach a maximum limit 
beyond which life could not exist. A point is easily con- 
ceivable where no force generated within the body of an 
animal — supposing one placed in such circumstances — 
would counterbalance the force of which it was, so to speak, 
cemented to the earth’s surface. All motion from place to 
place would then be absolutely impossible, and every move- 
ment within the animal system, save in a line tending to 
the earth’s centre, would be impossible likewise. The 
smallest and simplest animals would doubtless bear a greater 
gravitative force than the higher and larger. It is evident 
that they have, even now, a certain advantage in this respedt. 
Though under the influence of gravitation, as decidedly as 
are larger species, we see them — even when not specially 
organised for such feats — ascend smooth perpendicular sur- 
faces, and even travel along the lower side of a horizontal 
plane with their heads downwards. Naturalists have spent 
much time in explaining this faculty in the common house- 
fly, which, however, is only one out of thousands of species 
able to perform the same adtion. 
A fall, even from a height very great in proportion to their 
stature, seems to have no injurious effedt upon wingless 
insedts. This may at first sight seem due to the coat of 
armour, or so-called exo-skeleton, in which they are enclosed. 
But if a tortoise is dropped from a corresponding height its 
armour does not save it from damage. The reason must, 
then, be sought in the resistance offered by the cohe- 
sive power of the atmosphere to the fall of so light a 
substance. 
Even a relatively moderate increase in the force of gravi- 
tation might be produdtive of serious consequences to larger 
animals. In man, the downward pressure of the blood 
resulting from gravitation sometimes distends the veins of 
the feet and legs so as to produce a varicose condition. 
Were the human stature or the earth’s attradtion doubled, 
and still more if both these modifications took place con- 
