The Influence of Circumstances on the Actions and Habits of Animals

Concerning Speciation Among Living Things and the Idea Which We Should Attach to Speciation

Each species is influenced by circumstances over a long period.

• This leads to habits which exert influences on the parts of each individual of the species

These habits modify these parts to make them appropriate to the acquired habits.

A species is every collection of similar individuals produced by other individuals just like themselves.

We assume that:

• the individuals in a species never vary in their specific characteristics
• therefore the species has an absolute constancy in nature.

I contest this assumption.

People assume that:

• living things make up eternally distinct species because of their invariable characteristics
• the existence of these species is as ancient as nature herself

The assumption is contradicted everyday by those who have followed nature for a long time.

Naturalists are extremely embarrassed in their attempts to define the objects which they have to consider species.

There are collections of individuals who:

• resemble each other in their organic structure and in the totality of their parts.
• remain in the same condition generation after generation

The individuals of a species must perpetuate themselves without variation, as long as:

• the circumstances which influence their life do not essentially vary
• the existing prejudices agree with the successive regeneration of similar individuals

People have assumed that each species:

• did not vary
• was also as old as nature
• was uniquely created by the Supreme Author

The notion of species among living creatures which people formed was very simple.

• It seemed confirmed by the constancy in the apparent form of individuals which reproduction or generation perpetuated.

Such individuals create for us a great number of those alleged species which we see every day.

People create so many genera and species that the study and the definition of these species are now almost unworkable!

I do not believe that animals form a very simple series, equally modified throughout.

But I believe they form a branching series, with irregular gradations, something which has no discontinuity in its parts or which has not always had them, if it is true that as a result of some lost species such discontinuity occurs here and there.

It follows that species which end each branch of this general series have other closely related species which meld into them.

This well known state of things leads me now to provide an illustration.

I arrange the species in a series, placing them based on their natural affinities.

Then the generic and specific distinctions were very easy to establish.

But now that our collections are extremely profuse, if you follow the series which I have cited immediately above from the species you first chose right up to the one which you selected second (which is very different from the first), you will reach that second species through a series of slight modifications without having noticed distinctions worthy of attention.

What experimental zoologist or botanist has not explored this basis?

What a crowd of shell creatures the mollusks show us from all countries and all seas, eluding our ways of distinguishing them and wearing out our resources on this question.

Go back up to the fish, reptiles, birds, even to mammals.

You will see everywhere, apart from the gaps which still have to be filled, the modifications which link up neighbouring species, even genera, leaving hardly any places for our ingenuity to establish good distinctions.

And in its various parts does not botany, which focuses on the other series making up the plants, display exactly the same state of things?

What difficulties are not experienced nowadays in studying and determining species in the genera Lichen, Fucus, Carex, Poa, Piper, Euphorbia, Erica, Hieracium, Solanum, Geranium, Mimosa, and on and on?

When we formed these genera, we knew only a small number of their species; thus, they were easy to distinguish. But now that almost all the gaps between them have been filled, our specific differences are necessarily minute and very frequently insufficient.

What causes this?

The individuals of a species change their situation, climate, manner of life, or habits.

• These changes lead to influences which change gradually the constancy and the proportions of their parts, shape, faculties, even their organic structure.
• This leads to mutations

In the same climate, significantly different situations and exposures induce changes in the individuals.

But as time passes, those individuals reproduce successively.

The continual difference in the same circumstances leads to changes in them which become essential to their being.

After many generations, these individuals will be transformed into a new species.

For example, the seeds of a plant common to a humid prairie are transported by the wind to the slope of a neighbouring hill at a higher altitude.

• With each generation, it moves higher where it is more arid.
• These new conditions transform it into a new species.

The same thing happens to animals which are forced to change their climate, manner of life, and habits.

But these require even more time compared to plants.

People think that a species who have existed in the same form for a long time necessarily requires that its members cannot mate with those of another species.

Unfortunately, observation has demonstrated that this idea has no foundation.

Hybrids are very well known among plants.

The matings between individuals of very different species among animals prove that the limits between these species, supposedly constant, were not constant.

Often nothing results from these odd matings, especially when they involve very different types.

• The offspring are generally infertile.*

But when the disparity is less, the flaws do not occur.

This method by itself is sufficient to create varieties gradually which then become races, and which, in time, make up what we call species.