NATURAL SELECTION 6 



alluvial flat ; and lastly, it has been exposed to changes in its 

 conditions, being grown sometimes in one district and some- 

 times in another, in different soils. Under such circumstances, 

 scarcely a plant can be named, though cultivated in the rudest 

 manner, which has not given birth to several varieties. It can 

 hardly be mai'ntained that during the many changes which this 

 earth has undergone, and during the natural migrations of 

 plants from one land or island to another, tenanted by different 

 species, that such plants will not often have been subjected to 

 changes in their conditions analogous to those which almost 

 inevitably cause cultivated plants to vary. Xo doubt man 

 selects varying individuals, sows their seeds, and again selects 

 their varying offspring. But the initial variation on which 

 man works, and without which he can do nothing, is caused 

 by slight changes in the conditions of life, which must often 

 have occurred under nature. Man, therefore, may be said to 

 have been trying an experiment on a gigantic scale ; and it 

 is an experiment which nature during the long lapse of time 

 has incessantly tried. Hence it follows that the principles 

 of domestication are important for us. The main result 

 is that organic beings thus treated have varied largely, 

 and the variations have been inherited. This has ap- 

 parently been one chief cause of the belief long held by some 

 few naturalists that species in a state of nature undergo 

 change. 



I shall in this volume treat, as fully as my materials j^ermit, 

 the whole sulyect of variation under domestication. We may 

 thus hope to obtain some light, little though it be, on the 

 causes of variability, — on the laws which govern it, such as the 

 direct action of climate and food, the effects of use and disuse, 

 and of correlation of growth, — and on the amount of change to 

 which domesticated organisms are liable. We shall learn 

 something of the laws of inheritance, of the effects of crossing 

 different breeds, and on that sterility which often supervenes 

 when organic beings are removed from their natural 

 conditions of life, and likewise when they are too closely 

 interbred. During this investigation we shall see that the 

 principle of Selection is highly important. Although man does 

 not cause variability and cannot even i^reventit, he can select, 



B 2 



