The Effects of Geographical Distribution

Obserations on the distribution of organic beings

1. The similarity or dissimilarity between the inhabitants of various regions are not caused by their climatal and other physical conditions.

Of late, almost every author who has studied the subject has come to this conclusion.

This is proven by America.

The vast American continent has the most diversified conditions:

• the most humid districts
• arid deserts
• lofty mountains
• grassy plains, forests, marshes, lakes, and great rivers, under almost every temperature.

Despite this parallelism in the conditions of the Old and New Worlds, their living productions are widely different!

The southern hemisphere has extremely similar conditions, yet their faunas and floras are utterly dissimilar.

The productions of South America south of lat. 35 deg and those north of 25 deg of a different climate are more closely related than they are to the productions of Australia or Africa under nearly the same climate.

The same could be said about the inhabitants of the sea.

2. Barriers to free migration correlate to the differences between the productions of various regions.

This is seen in the great difference between the productions of the New and Old Worlds, except in the northern parts where the land almost joins.

• This could have caused free migration for the northern temperate forms

The same is true for the great difference between the inhabitants of Australia, Africa, and South America under the same latitude.

These countries are isolated from each other.

We find different productions on the opposite sides of:

• lofty mountain-ranges
• great deserts
• large rivers

The sea follows the same law.

The marine faunas of South and Central America in the east and west are most distinct.

The Pacific has no impassable barriers.

And so many fish and shells range from the Pacific into the Indian Ocean and are common to the eastern islands of the Pacific and the eastern shores of Africa, on almost exactly opposite meridians of longitude.

3. The affinity of the productions of the same continent or sea, though the species themselves are distinct at different points and stations.

It is a law of the widest generality, and every continent offers innumerable instances. Neverthelessthe naturalist in travelling, for instance, from north to south never fails to be struck by the manner in which successive groups of beings, specifically distinct, yet clearly related, replace each other.

He hears from closely allied, yet distinct kinds of birds, notes nearly similar, and sees their nests similarly constructed, but not quite alike, with eggs coloured in nearly the same manner.

The plains near the Straits of Magellan are inhabited by a species of Rhea (American ostrich).

Northward, the plains of La Plata has another species of the same genus.

These are different from those found in Africa and Australia under the same latitude.

On these same plains of La Plata, we see the agouti and bizcacha. These:

• have the same habits as our hares and rabbits
• belong to the same order of Rodents

But they display an American type of structure.

We ascend the lofty peaks of the Cordillera and we find an alpine species of bizcacha; we look to the waters, and we do not find the beaver or musk-rat, but the coypu and capybara, rodents of the American type.

We may look back to past ages, as shown in the last chapter, and we find American types then prevalent on the American continent and in the American seas. We see in these facts some deep organic bond, prevailing throughout space and time, over the same areas of land and water, and independent of their physical conditions.

The naturalist must feel little curiosity, who is not led to inquire what this bond is.

This bond, on my theory, is simply inheritance.

• It alone produces organisms quite like each other.

The dissimilarity of the inhabitants of different regions is from:

• modification through natural selection.
• the direct influence of different physical conditions.

The degree of dissimilarity will depend on:

• the migration of the more dominant forms of life from one region into another having been effected with more or less ease, at periods more or less remote
• the nature and number of the former immigrants
• their action and reaction, in their mutual struggles for life;–the relation of organism to organism being, as I have already often remarked, the most important of all relations.

Thus the high importance of barriers comes into play by checking migration; as does time for the slow process of modification through natural selection.

Widely-ranging species, abounding in individuals, which have already triumphed over many competitors in their own widely-extended homes will have the best chance of seizing on new places, when they spread into new countries.

In their new homes they will be exposed to new conditions, and will frequently undergo further modification and improvement; and thus they will become still further victorious, and will produce groups of modified descendants.

On this principle of inheritance with modification, we can understand how it is that sections of genera, whole genera, and even families are confined to the same areas, as is so commonly and notoriously the case. I believe, as was remarked in the last chapter, in no law of necessary development.

The variability of each species:

• is an independent property.
• will be taken advantage of by natural selection, only so far as it profits the individual in its complex struggle for life

Likewise, the degree of modification in different species will be no uniform quantity.

If, for instance, a number of species, which stand in direct competition with each other, migrate in a body into a new and afterwards isolated country, they will be little liable to modification; for neither migration nor isolation in themselves can do anything.

These principles come into play only by bringing organisms into new relations with each other, and in a lesser degree with the surrounding physical conditions.

As we have seen in the last chapter that some forms have retained nearly the same character from anenormously remote geological period, so certain species have migrated over vast spaces, and have not become greatly modified.