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Show 270 CAUSES OF V AlUABILITY. CHAP. XXII. ditions, at least in a visible manner, than the female element or ovule; and we know from Gartner's and Wichura's statements that a hybrid used as the father and crossed with a pure species gives a greater degree of variability to the offspring, than does the same hybrid when "':sed as the mo~her. Lastly, it is certain that variability may be ~ran~m1tted throug~ either sexual element, whether or not originally excited m then:, ~or Kolreuter and Gartner sJ found that when two species were crossed, if either one was variable, the offspring were rendered variable. Summary.- From the facts given in this chapter, we may conclude that the variability of organic beings under domestication, although so general, is not an inevitable contingent on growth and reproduction, but results from the conditions to which the parents have been exposed. Changes of any kind in the conditions of life, even extremely slight changes, often suffice to cause variability. Excess of nutriment is perhaps the most efficient single exciting cause. Animals and plants continue to be variable for an immense period after their first domestication; but the conditions to which they are exposed never long remain quite constant. In the course of time they can be habituated to certain changes, so as to become less variable; and it is possible that when first domesticated they may have been even more variable than at present. There is good evidence that the power of changed conditions accumulates ; so that two, three, or more generations must be exposed to new conditions before any effect is visible. The crossing of distinct forms, which have already become variable, increases in the offspring -the tendency to further variability, by the unequal commingling of the characters of the two parents, by the reappearance of long-lost characters, and by the appearance of absolutely new characters. Some variations are induced by the direct action of the surrounding conditions on the whole organisation, or on certain parts alone, and other variations are induced indirectly through the reproductive system being affected in the same manner as is so common with organic beings when removed from their natural conditions. The causes which induce variability act on the mature organism, on the embryo, and, as we have good reason to believe, on both sexual elements before impregnation has been effected. 53 ' Dritte Fortsetzung,' &r., s. 123; 'Bastarderzeugung,' s. 249. CHAP. XXilf. DEI<'INITE ACTION OF CONDITIONS OF LIFE. 271 CHAPTER XXIII. DIRECT AND DEFINITE ACTION OF THE EXTERNAL CONDITIONS OF LIFE. SLIGHT MODIFICATIONS IN PLANTS FROJ\I THE DEFINITE ACTION OF CHANGED CONDITIONS, IN SIZE, COLOUR, CHEMICAL PROPERTIES, AND IN THE STATE OF THE TISSUES - LOCAL DISEASES - CONSPICUOUS l\IODIFICATIONS FROM CHANGED CLillfATE OR FOOD, ETC.- PLUXAGE OF BIRDS AFFECTED BY PECULIAR NUTRIMENT, AND BY THE INOCULATION OF POISON- LAJ\"'D-SHEJ,LS- MODIFICATIONS OF ORGANIC BEINGS L.~ A STATE OF NATURE THROUGH THE DEFINITE ACTION OF EXTERNAL CONDITIONS- COMPARISON OF AMERICAN AND EUROPEAN TREES- GALLS- EFFECTS OF PARASITIC FUNGI- CONSIDERATIONS OPPOSED TO THE BELIEF IN THE POTEN'l' lNFLUENCE OF CHANGED EXTERNAL CONDITIONS- PARALLEL SERIES OF VARIETIES - Al\IOUNT OF VARIATION DOES NOT CORRESPOND WITH THE DEGREE OF CHANGE IN 'fHE CONDITIONS- BUD-VAHIATION- MONSTROSITIES PHODUCED BY UNNATURAL TUEATMENT- SUMMARY. IF we ask ourselves why this or that character has been modified under domestication, we are, in most cases lost in utter darkness. Many naturalists, especially of the French school, attribute every modification to the "monde ambiant," that is, to ehanged climate, with all its diversities of heat and cold, dampness and dryness, light and electricity, to the nature of the soil, and to varied kinds and amount of food. By the term definite action, as used in this chapter, I mean an action of such a nature that, when many individuals of the same variety are exposed during several generations to any change in their physical conditions of life, all, or nearly all the individuals, are modified in the same manner. A new sub-variety would thus be produced without the aid of selection. I do not include under the term of definite action the effects of habit or of the increased use and disuse of various organs. l'IIoclifications of this nature, no doubt, are definitely caused by the conditions to which the beings are subjected; but they depend much less on the nature ofthe conditions than on the laws of growth; hence they are included under a distinct head in the |