The Geometric Increase in Population

The rule is that every organic being naturally increases at such a high rate. If not destroyed, the earth would be covered by the progeny of a single pair.

There is no exception to this.

Even slow-breeding man has doubled in 25 years. At this rate, in a few thousand years, there would be no room for his progeny.

Linnaeus has calculated that if an annual plant produced only 2 seeds and their seedlings next year produced 2, and so on, then in 20 years there would be a million plants.

The elephant is the slowest breeder of all known animals.

It breeds when 30 years old until 90.

It brings forth 3 pairs of young in this interval.

So that at the end of 500 years, there would be 15,000,000 elephants descended from the first pair.

This rule is proven by the numerous cases of the rapid increase of wild animals when circumstances have been favourable to them for 3 consecutive seasons.

This is also proven by cattle and horses which have run wild.

Plants have been introduced and have spread throughout whole islands in less than 10 years.

The conditions of life have been very favourable. Consequently, nearly all the young have been enabled to breed.

All plants and animals tend to increase at a geometrical ratio. This increase must be checked by destruction at some period of life.

Our familiarity with the larger domestic animals tends to mislead us because we do not see them destroyed. We forget that:

• thousands are annually slaughtered for food
• in the wild, an equal number would be destroyed

The condor lays 2 eggs. The ostrich lays 20.

Yet in the same country, the condor might outnumber the ostrich.

The Fulmar petrel lays but one egg. Yet it is believed to be the most numerous bird in the world.

One fly deposits hundreds of eggs. The hippobosca lays just one.

But this difference does not determine how many individuals of the two species can be supported in a district.

Those species depend on a rapidly fluctuating amount of food.

• Having many eggs is of some importance to them as it allows them rapidly to increase in number.

But the real importance of a large number of eggs or seeds is to make up for much destruction at some period of life.

• This period in the great majority of cases is an early one.

If an animal can in any way protect its own eggs or young, a small number may be produced.

Yet the average stock is fully kept up. But if many eggs or young are destroyed, many must be produced, or the species will become extinct.

If a tree lived for 1,000 years, it would be enough for it to produce 1 seed per 1,000 years to keep its numbers up suppose that this seed were never destroyed.

In looking at Nature, it is most necessary to keep the foregoing considerations always in mind–

Never forget that:

• every organic being:
  • is striving to the utmost to increase in numbers
  • lives by a struggle at some period of its life
• heavy destruction inevitably falls either on the young or old, during each generation or at recurrent intervals.
• the population will almost instantaneously increase to any amount if
  • any checks are lightened
  • the destruction is mitigated ever so little

The face of Nature is like a yielding surface with 10,000 sharp wedges packed close together and driven inwards by incessant blows, sometimes one wedge being struck, and then another with greater force.

It is unclear what checks the natural tendency of each species to increase.

The most vigorous species swarms in numbers.

• We do know not exactly what checks its population.

The Destruction of Individuals

Eggs or very young animals seem generally to suffer most, but this is not invariably the case.

With plants there is a vast destruction of seeds, but, from some observations which I have made, I believe that it is the seedlings which suffer most from germinating in ground already thickly stocked with other plants.

Seedlings are destroyed in vast numbers by various enemies.

On a ground 3 x 2 feet, dug and cleared, free from other plants, I marked all the seedlings of our native weeds as they came up.

Out of the 357, 295 were destroyed, chiefly by slugs and insects.

If turf which has long been mown, and the case would be the same with turf closely browsed by quadrupeds, be let to grow, the more vigorous plants gradually kill the less vigorous, though fully grown, plants: thus out of 20 species growing on a little plot of turf (three feet by four) nine species perished from the other species being allowed to grow up freely.

The amount of food for each species of course gives the extreme limit to which each can increase; but very frequently it is not the obtaining food, but the serving as prey to other animals, which determines the average numbers of a species.

Thus, the stock of partridges, grouse, and hares on any large estate depends chiefly on the destruction of vermin.

If not one head of game were shot during the next twenty years in England, and, at the same time, if no vermin were destroyed, there would, in all probability, be less game than at present, although hundreds of thousands of game animals are now annually killed.

On the other hand, in some cases, as with the elephant and rhinoceros, none are destroyed by beasts of prey: even the tiger in India most rarely dares to attack a young elephant protected by its dam.

Climate plays an important part in determining the average numbers of a species, and periodical seasons of extreme cold or drought, I believe to be the most effective of all checks.

I estimated that the winter of 1854-55 destroyed 4/5 of the birds in my own grounds.

10% is an extraordinarily severe mortality from epidemics with man.

The action of climate seems to be independent of the struggle for existence by reducing food.

When we travel from south to north, or from a damp region to a dry, we see some species gradually getting rarer and rarer, and finally disappearing. We are tempted to attribute the whole effect to the change of climate.

But this is a very false view because each species is constantly suffering enormous destruction at some period of its life, from enemies or from competitors for the same place and food.

If these enemies be are favoured by the change of climate, they will increase in numbers.

• Since each area is already full with inhabitants, the other species will decrease.

When we travel southward and see a species decreasing in numbers because they are hurt by the favored species.

So it is when we travel northward, but in a lesser degree. This is because the number of species of all kinds, and therefore of competitors, decreases northwards.

Hence in going northward, or in ascending a mountain, we far oftener meet with stunted forms, due to the directly injurious action of climate, than we do in proceeding southwards or in descending a mountain.

In the following regions, the struggle for life is almost exclusively with the elements:

• the Arctic regions, or snow-capped summits, or
• absolute deserts

Our gardens have imported plants which can well endure our climate. But they cannot:

• compete with our native plants
• resist destruction by our native animals.

This proves that climate acts in main part indirectly by favouring other species.