Other Species as a Check

Plants and animals, most remote in the scale of nature, are bound together by a web of complex relations.

The exotic Lobelia fulgens, in this part of England, is never visited by insects.

Consequently, it can never set a seed.

Many of our orchidaceous plants require the visits of moths to:

• remove their pollen-masses and
• fertilise them.

Bumble-bees are indispensable to the fertilisation of the heartsease (Viola tricolor) because other bees do not visit this flower.

From experiments which I have tried, I have found that the visits of bees, if not indispensable, are at least highly beneficial to the fertilisation of our clovers.

But bumble-bees alone visit the common red clover (Trifolium pratense), as other bees cannot reach the nectar.

If the whole genus of bumble-bees became extinct or very rare in England, the heartsease and red clover would become very rare, or wholly disappear.

The number of humble-bees in any district depends in a great degree on the number of field-mice, which destroy their combs and nests.

Mr. H. Newman, who has long attended to the habits of humble-bees, believes that ‘more than two thirds of them are thus destroyed all over England.’

The number of mice is largely dependent, as every one knows, onthe number of cats; and Mr. Newman says, ‘Near villages and small towns I have found the nests of humble-bees more numerous than elsewhere, which I attribute to the number of cats that destroy the mice.’

The presence of a feline animal in large numbers in a district might determine the frequency of certain flowers in that district through the intervention first of mice and then of bees!

When we look at the plants and bushes clothing an entangled bank, we are tempted to attribute their proportional numbers and kinds to chance.

But this is wrong.

When an American forest is cut down, a very different vegetation springs up.

But it has been observed that the trees now growing on the ancient native American mounds, in the Southern United States, display the same beautiful diversity and proportion of kinds as in the surrounding virgin forests.

What a struggle between the several kinds of trees must here have gone on during long centuries, each annually scattering its seeds by the thousand;

what war between insect and insect–between insects, snails, and other animals with birds and beasts of prey–

all striving to increase, and all feeding on each other or on the trees or their seeds and seedlings, or on the other plants which first clothed the ground and thus checked the growth of the trees!

Throw up a handful of feathers and all must fall to the ground according to definite laws.

But how simple is this problem compared to the action and reaction of the innumerable plants and animals which have determined, in the course of centuries, the proportional numbers and kinds of trees now growing on the old Indian ruins!

The dependency of one organic being on another, as of a parasite on its prey, lies generally between beings remote in the scale of nature.

This is often the case with those which struggle with each other for existence, as in the case of locusts and grass-feeding quadrupeds.

But the struggle almost invariably will be most severe between the individuals of the same species, for they frequent the same districts, require the same food, and are exposed to the same dangers.

In the case of varieties of the same species, the struggle will generally be almost equally severe.

The best varieties of wheat are those which best suit the soil or climate and are naturally the most fertile.

If several varieties of wheat are sown together, and the mixed seed is resown, some of the best varieties will beat the others and so yield more seed.

They will consequently supplant the other varieties in a few years.

To keep up a mixed stock of such close varieties as the variously coloured sweet-peas, each year:

• they must be harvested separately
• the seed should then be mixed in due proportion

Otherwise, the weaker kinds will steadily decrease in numbers and disappear.

This also applies with the varieties of sheep.

Certain mountain-varieties will starve out other mountain-varieties, so that they cannot be kept together.

The same result has followed from keeping together different varieties of the medicinal leech.

Our domestic plants or animals do not have the same strength, habits, and constitution.

The original proportions of a mixed stock could not be kept up for 6 generations if:

• they were allowed to struggle together like wild organisms, and
• the seed or young were not annually sorted.

The species of the same genus usually have some similarity in habits and constitution, and always in structure.

The struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinct genera.

We see this in the recent extension over parts of the United States of one species of swallow having caused the decrease of another species.

The recent increase of the missel-thrush in parts of Scotland has caused the decrease of the song-thrush.

How frequently we hear of one species of rat taking the place of another species under the most different climates! In Russia the small Asiatic cockroach has everywhere driven before it its great congener.

One species of charlock will supplant another, and so in other cases.

We can dimly see why the competition should be most severe between allied forms, which fill nearly the same place in the economy of nature; but probably in no one case could we precisely say why one species has been victorious over another in the great battle of life.