Naturalization

The greatest amount of life can be supported by great diversification of structure.

There is always great diversity in extremely small areas:

• that are open to immigration, and
• where the contest is severe

A 3 x 4 feet piece of turf ad been exposed for many years to exactly the same conditions.

• It supported 20 species of plants belonging to 18 genera and to 8 orders
• This shows how they differed from each other.

So it is with the plants and insects on small and uniform islets; and so in small ponds of fresh water.

Farmers can raise most food by a rotation of plants belonging to the most different orders.

This is called a simultaneous rotation.

Most of the animals and plants which live close round any small piece of ground, could live on it (supposing it not to be in any way peculiar in its nature), and may be said to be striving to the utmost to live there;

But where they come into the closest competition with each other, the advantages of diversification of structure, with the accompanying differences of habit and constitution, determine that the inhabitants, which thus jostle each other most closely, shall, as a general rule, belong to what we call different genera and orders.

The same principle is seen in the naturalisation of plants through man’s agency in foreign lands.

It might have been expected that the plants which have succeeded in becoming naturalised in any land would generally have been closely allied to the indigenes; for these are commonly looked at as specially created and adapted for their own country.

It might, also, perhaps have been expected that naturalised plants would have belonged to a few groups more especially adapted to certain stations in their new homes. But the case is very different; and Alph. De Candolle has well remarked in his great and admirable work, that floras gain by naturalisation, proportionally with the number of the native genera and species, far more in new genera than in new species.

Dr. Asa Gray’s ‘Manual of the Flora of the Northern United States’ has 260 naturalised plants, of a highly diversified nature, belonging to 162 genera.

They differ from the indigenes.

Out of the 162 genera, 100 genera are not indigenous.

Thus a large proportional addition is made to the genera of these States.

The natives would have had modification to gain an advantage over the other natives.

I infer that the diversification of structure, amounting to new generic differences, would have been profitable to them.

The advantage of diversification in the inhabitants of the same region is the same as that of the physiological division of labour in the organs of the same individual body–a subject so well elucidated by Milne Edwards.

A stomach draws the most nutriment by being adapted to digest matter.

It follows that the more widely and perfectly the animals and plants are diversified for the habits of life, so will more individuals can support themselves.

A set of undiversified animals could hardly compete with a diversified set.

Australian marsupials are divided into groups differing little from each other.

They somewhat match our carnivorous, ruminant, and rodent mammals which they could not successfully compete with these well-pronounced orders.

This is because the diversification of Australian mammals are in an early and incomplete stage of development.

The modified descendants of any one species will succeed by so much the better as they become more diversified in structure, and are thus enabled to encroach on places occupied by other beings.