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Show 14 V .A.RIATION UNDER DOMESTICATION. [Cn..&.P I. • CI-IAPTER I . VARIATION UNDER DOMESTICATION. Causes of Variability-Effects of Habit-Correlation of Growth-Inheritance- Character of Domestic Varieties-Difficulty of distinguishing between Varieties and Species-Origin of Domestic Varieties from one or more Species-Domestic Pigeons, their Dfferences and Origin-Principle of Selection anciently followed, its Effects-Methodical and Unconscious Selection- Unknown Origin of our Domestic Productions-Circumstances favourable to Man'~ power of Selection. WrrEN we look to the individuals of the san1e variety or sub-variety of our older cultivated plants and animals, one of the first points which strikes us, is, that they generally differ 1nuch more from each other, than do the individuals of any one species or variety in a state of nature. When we reflect on the vast diversity of the plants and animals which have been cultivated, and which have varied during all as-es under the 1nost different climates and treatment, I tlnnk we are driven to conclude that this greater variability is simply 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 have been exposed under nature. There is, also, I think, son1e probability in the view propounded by Andrew l{night, that this variability may be partly connected with excess of food. It see1ns pretty clear that organic beings must be exposed during several generations to the new conditions of life to cause any appreciable amount of variation ; and that when the organ· isation has once begun to vary . it generally continues to vary for many generations. No case is on record of a variable being ceasing to be variable under cultivation. O.ur oldest cultivated plants, such as wheat, still often yield new varieties : our oldest domesticated animals are still capable of rapid improvement or modification. CrrAP. I.J UNDER DOMESTICATION. 15 It ~as .b.een disputed at what period of life, the causes of vanabiht:y, whatever they may be, generally act; whether dunng the earll or late period o~ development of the embryo, or at the Instant of conception. Geoffroy St. IIilaire's experiments show that unnatural treatment of the en1bryo causes Inonstrosities ; and monstrosities cannot be separated by any clear line of distinction from mere variations. But I am strongly inclined to suspect that the most frequent cause of variability may be attributed to the male and female reproductive elements ha~ing been affected prior to the act of conception. Several r~sons Inake me believe in this ; but the chief one is the remarkable effect which confinement or cultivation has on the .functions of the reproductive system ; this system appeanng to ?e ~ar more susceptible than any other part of the organisation, to the action of any change in the COJ?-ditions of life. ~ othing is more easy than to tame an animal, and few th1ngs more difficult than to get it to breed freely under confinement, even in the many cases when the male and female unite. 1-Iow many animals there are which will not br~ed, t~ough_living long under ~ot very close confinement In their native country ! This Is genera1ly attributed to vitiated instincts· but how many cultivated plants display the utmost ~igour, and yet rarely or never seed! In some few such cases it has been found out that very trifling changes, such as a little more or less water at some particular period of growth ·will determine whether or not the plant sets a seed. i cannot here enter on the copious details which I have collected on this curious subject; but to show. how singular the laws are which determine the reproduction of animals un~er confinement, I may just mention that carnivorous anJmals, even from the tropics, breed in this country· pretty freely under cofinen:ent, with the exception of the plantigrades or bear family; whereas, carnivorous birds with the 1:arest exceptions, hardly ever lay fertile eggs. Many exotlc pla~t~ have. pollen utterly worthless, in the same exact condition as In the most sterile hybrids. When on the one hand, we see do:nesticated ani~als and pl~nts, though often weak and sickly, yet breeding quite freely |