106 
PROCEEDINGS OF THE PERTHSHIRE SOCIETY OF NATURAL SCIENCE, 
other animals. These dangers, proceeding from extreme 
sources, form the key-note, so to speak, of our subject, ■ 
bringing about a fight for life which extends to all animals ! 
in a state of nature, and supplying us with the immediate ! 
cause, some of the effects of which are to be seen in the 
cases I shall bring under your notice. But at the outset, 
and as introductory to our subject, allow me to refer to the 
competition for life to which all animate nature is exposed. 
The power of competition is not only experienced by the 
lower animals, but man himself comes under its sway. It , 
was competition, and the difficulty of obtaining food for j 
their ever-increasing lumbers, that led barbarous nations j 
to resort to the horrible practice of infanticide; so that, 
by the reduction of their numbers, the struggle for life > 
might not be so keenly felt by those remaining. It is 
competition which, to a large extent, regulates the accumu¬ 
lation and distribution of wealth; and it is competition » 
which has effected the onward progress of civilization, | 
culminating in the wonderful advances of scientific and 
commercial pursuits during the present century. Among ' t 
animals, too, as we have previously noticed, it is the most j 
important agent in maintaining equally poised the balance 1 
of Nature. When we consider, on the one hand, the rate 
of increase of which many insects are capable, and, on the ] 
other, the number which annually perish—often, I sup- I 
pose, being as many as are annually born—we can 
form pretty clear ideas of how keen the struggle | 
must be. The productiveness or fertility of insects is 
often enormous. Many leave eggs to be numbered 
by the hundred, some by the thousand. Others leave 
larger numbers. The common house-fly, for instance, often 
leaves a progeny of 20,000. Among aphides, however, we 
have perhaps the most remarkable fecundity of any. We 
all know by experience how rapidly the green fly multiplies 
on our rose trees and geraniums. Reaumer calculates that 
oue aphis may be the mother of the enormous number of 
3,904,900,000 individuals during the month or six weeks of 
her existence, and Professor Huxley has made the curious 
calculation that, assuming an aphis to weigh as little as the 
one-thousandth part of a grain, and that it requires a man 
to be very stout to weigh more than 2,000,000 grains, he 
shews that the tenth brood of aphides alone, without adding 
the products of all the generations which precede the tenth, 
if all the members survive the perils to which they are ex. 
posed, would contain more ponderable substance than 
500,000,000 of stout men—that is, more than the whole 
population of China. As, however, it is impossible for us 
to grasp such vast numbers, let us take a similar case, 
though one a little more easy of appreciation. Suppose a 
butterfly, or moth, leave the very moderate number of 10 
eggs, which, in due time, develope into 5 pairs of insects_ 
Now, if we assume that these increase at the same rate, and 
that the conditions surrounding them are perfectly favour¬ 
able, so that none perish, we shall find that the tenth 
generation, as the progeny of this one insect, will number 
nearly 20,000,000 individuals. When we consider figures 
such as these, we can see what disastrous results would 
follow were insects allowed to multiply, even at such 
a low rate of increase as I have just referred to, with¬ 
out any checks. However, we find that, just as a 
large proportion of the seeds of the vegetable kingdom are 
not allowed to continue their kind, but serve as food for 
many animals, so vast numbers of insects are preyed upon 
by other predatory insects and by birds. 
We are thus led briefly to notice the close connec¬ 
tion and dependence of one animal upon another for 
its subsistence. Thus, one insect in a district, may 
be the cause of the presence there of some predacious 
insect: this, again, causes the presence of insectivorous 
birds. We see how birds follow in the wake of insects, 
though not with reference to any district, in the case of 
the swallow. It has become proverbial of fine weather 
when the swallows fly high, and as an omen of foul 
when their flight is low. But why is it that the swallow 
flies high or low ? Not because it recognises the rarety or 
density of the atmosphere. No ; but for the simple reason 
that the insects upon which they feed, and which they 
therefore follow so closely, are regulated in their flight by 
the conditions of the air. We find, then, that to keep some 
insects from too rapid increase, predatory insects, or some 
opposing agents, are necessary to hold them in check. These, 
again, need some restriction, else they too might increase, 
so as to overcome, and perhaps exterminate, the former 
ones, and thus, owing to our imperfect knowdedge of, and our 
inability to follow, the interchangeableness and intricacy 
I of the laws regulating the distribution of species, and the 
I intimate connection existing throughout the whole organic 
* world, we become quite confused, and cannot see our way 
amid such a labyrinth. Sir Darwin, for instance, tells us 
that the fertilisation of the Dutch clover depends on the 
cat. You will wonder how he arrives at such a conclusion, 
but his line of reasoning runs thus : the humble bee is the 
only insect which fertilises this clover; field mice 
destroy the nests and combs of the humble bee ; 
i and they in their turn are destroyed by cats. Thus the 
abundance or otherwise of this clover in a district may be 
materially affected by the number of cats. A later writer, 
| in seeking to go a step further back, rather dryly suggests 
I that this useful animal again, in its turn, may owe its 
! abundance to the number of unmarried ladies of mature 
