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04 - Natural Selection 04-12 - On the Degree to which Organisation tends to advance 90 The three lowest orders of mammals, namely, marsupials, edentata, and rodents, co-exist in South America in the same region with numerous monkeys, and probably interfere little with each other.

kangaroo
kangaroo

armadillo
armadillo

rat
rat

monkey
monkey

South America
South America


Although organisation, on the whole, may have advanced and be still advancing throughout the world, yet the scale will always present many degrees of perfection; for the high advancement of certain whole classes, or of certain members of each class, does not at all necessarily lead to the extinction of those groups with which they do not enter into close competition.

In some cases, as we shall hereafter see, lowly organised forms appear to have been preserved to the present day, from inhabiting confined or peculiar stations, where they have been subjected to less severe competition, and where their scanty numbers have retarded the chance of favourable variations arising.
05 - Laws of Variation 05-12 - Reversion to Long Lost Characters 90 What now are we to say to these several facts?

We see several distinct species of the horse-genus becoming, by simple variation, striped on the legs like a zebra, or striped on the shoulders like an ass. In the horse we see this tendency strong whenever a dun tint appears- a tint which approaches to that of the general colouring of the other species of the genus.

horse
horse

ass
ass

Dun Horse
Dun Horse


The appearance of the stripes is not accompanied by any change of form or by any other new character.

We see this tendency to become striped most strongly displayed in hybrids from between several of the most distinct species.

Now observe the case of the several breeds of pigeons: they are descended from a pigeon (including two or three sub-species or geographical races) of bluish colour, with certain bars and other marks; and when any breed assumes by simple variation a bluish tint, these bars and other marks invariably reappear; but without any other change of form or character.

pigeon
pigeon


When the oldest and truest breeds of various colours are crossed, we see a strong tendency for the blue tint and bars and marks to reappear in the mongrels.

I have stated that the most probable hypothesis to account for the reappearance of very ancient characters, is- that there is a tendency in the young of each successive generation to produce the long-lost character, and that this tendency, from unknown causes, sometimes prevails.

And we have just seen that in several species of the horse-genus the stripes are either plainer or appear more commonly in the young than in the old.

Call the breeds of pigeons, some of which have bred true for centuries, species; and how exactly parallel is the case with that of the species of the horse-genus!

horse
horse

pigeon
pigeon


For myself, I venture confidently to look back thousands on thousands of generations, and I see an animal striped like a zebra, but perhaps otherwise very differently constructed, the common parent of our domestic horse (whether or not it be descended from one or more wild stocks), of the ass, the hemionus, quagga, and zebra.

horse
horse

ass
ass

hemionus
hemionus

quagga
quagga

zebra
zebra
06 - Difficutiles in Theory 06-12 - Organs not in all Cases Absolutely Perfect 90 How the sense of beauty in its simplest form- that is, the reception of a peculiar kind of pleasure from certain colours, forms, and sounds- was first developed in the mind of man and of the lower animals, is a very obscure subject.

The same sort of difficulty is presented, if we enquire how it is that certain flavours and odours give pleasure, and others displeasure.

Habit in all these cases appears to have come to a certain extent into play; but there must be some fundamental cause in the constitution of the nervous system in each species.
06 - Difficutiles in Theory 06-03 - Absence or Rarity of Transitional Varieties 90 Thirdly, when two or more varieties have been formed in different portions of a strictly continuous area, intermediate varieties will, it is probable, at first have been formed in the intermediate zones, but they will generally have had a short duration.

For these intermediate varieties will, from reasons already assigned (namely from what we know of the actual distribution of closely allied or representative species, and likewise of acknowledged varieties), exist in the intermediate zones in lesser numbers than the varieties which they tend to connect.

From this cause alone the intermediate varieties will be liable to accidental extermination; and during the process of further modification through natural selection, they will almost certainly be beaten and supplanted by the forms which they connect; for these from existing in greater numbers will, in the aggregate, present more varieties, and thus be further improved through natural selection and gain further advantages.