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06 - Difficutiles in Theory 06-06 - Species with Habits Widely Diffferent from those of their Allies 10 Petrels are the most aerial and oceanic of birds, but in the quiet sounds of Tierra del Fuego, the Puffinuria berardi, in its general habits, in its astonishing power of diving, in its manner of swimming and of flying when made to take flight, would be mistaken by any one for an auk or a grebe; nevertheless it is essentially a petrel, but with many parts of its organisation profoundly modified in relation to its new habits of life; whereas the woodpecker of La Plata has had its structure only slightly modified.

In the case of the waterouzel, the acutest observer by examining its dead body would never have suspected its subaquatic habits; yet this bird, which is allied to the thrush family, subsists by diving- using its wings under water, and grasping stones with its feet.

Tierra del Fuego
Tierra del Fuego


All the members of the great order of hymenopterous insects are terrestrial excepting the genus Proctotrupes, which Sir John Lubbock has discovered to be aquatic in its habits; it often enters the water and dives about by the use not of its legs but of its wings, and remains as long as four hours beneath the surface; yet it exhibits no modification in structure in accordance with its abnormal habits.
06 - Difficutiles in Theory 06-03 - Absence or Rarity of Transitional Varieties 10 But, as by this theory innumerable transitional forms must have existed, why do we not find them embedded in countless numbers in the crust of the earth?

It will be more convenient to discuss this question in the chapter on the Imperfection of the Geological Record; and I will here only state that I believe the answer mainly lies in the record being incomparably less perfect than is generally supposed.

The crust of the earth is a vast museum; but the natural connections have been imperfectly made, and only at long intervals of time.

05 - Laws of Variation 05-13 - Summary 10 Our ignorance of the laws of variation is profound.

Not in one case out of a hundred can we pretend to assign any reason why this or that part has varied.

But whenever we have the means of instituting a comparison, the same laws appear to have acted in producing the lesser differences between varieties of the same species, and the greater differences between species of the same genus.

Changed conditions generally induce mere fluctuating variability, but sometimes they cause direct and definite effects; and these may become strongly marked in the course of time, though we have not sufficient evidence on this head.

Habit in producing constitutional peculiarities and use in strengthening and disuse in weakening and diminishing organs, appear in many cases to have been potent in their effects.

Homologous parts tend to vary in the same manner, and homologous parts tend to cohere.

Modifications in hard parts and in external parts sometimes affect softer and internal parts.

When one part is largely developed, perhaps it tends to draw nourishment from the adjoining parts; and every part of the structure which can be saved without detriment will be saved.

Changes of structure at an early age may affect parts subsequently developed; and many cases of correlated variation, the nature of which we are unable to understand, undoubtedly occur.

Multiple parts are variable in number and in structure, perhaps arising from such parts not having been closely specialised for any particular function, so that their modifications have not been closely cheeked by natural selection.

It follows probably from this same cause, that organic beings low in the scale are more variable than those standing higher in the scale, and which have their whole organisation more specialised.

Rudimentary organs, from being useless, are not regulated by natural selection, and hence are variable.

Specific characters- that is, the characters which have, come to differ since the several species of the same genus branched off from a common parent- are more variable than generic characters, or those which have long been inherited, and have not differed from this same period.
01 - Variations Under Domestication 01-01 - Causes of Variability 10 WHEN we compare the individuals of the same variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us is, that they generally differ more from each other than do the individuals of any one species or variety in a state of nature.
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And if we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all ages under the most different climates and treatment, we are driven to conclude that this great variability is due to our domestic productions having been raised under conditions of life not so uniform as, and somewhat different from, those to which the parent species had been exposed under nature.

There is, also, some probability in the view propounded by Andrew Knight, that this variability may be partly connected with excess of food.
Thomas Andrew Knight
Thomas Andrew Knight

It seems clear that organic beings must be exposed during several generations to new conditions to cause any great amount of variation; and that, when the organization has once begun to vary, it generally continues varying for many generations.

No case is on record of a variable organism ceasing to vary under cultivation.

Our oldest cultivated plants, such as wheat, still yield new varieties: our oldest, domesticated animals are still capable of rapid improvement or modification.