Additional cases of plants sterile with pollen from the same plant. 149 XVIII Mr. Sclater on the infertility of animals under confinement. 152 XVIII The Aperea a distinct species from the Guinea-pig. 230 XXI Prof. Jäger on hawks killing light-coloured pigeons. 273 XXIII Prof. Wisemann on the effects of isolation in the development of species. 281 XXIII The direct action of the conditions of life in causing variation. 317 XXIV Mr. Romanes on rudimentary parts. 324 to 328 XXV Some additional cases of correlated variability. 339 XXVI On Geoffrey St. Hilaire’s law of _“soi pour soi.”_ 357 to 404 XXVII The chapter on Pangenesis has been largely altered and re-modelled; but the essential principles remain the same. INTRODUCTION The object of this work is not to describe all the many races of animals which have been domesticated by man, and of the plants which have been cultivated by him; even if I possessed the requisite knowledge, so gigantic an undertaking would be here superfluous. It is my intention to give under the head of each species only such facts as I have been able to collect or observe, showing the amount and nature of the changes which animals and plants have undergone whilst under man’s dominion, or which bear on the general principles of variation. In one case alone, namely in that of the domestic pigeon, I will describe fully all the chief races, their history, the amount and nature of their differences, and the probable steps by which they have been formed. I have selected this case, because, as we shall hereafter see, the materials are better than in any other; and one case fully described will in fact illustrate all others. But I shall also describe domesticated rabbits, fowls, and ducks, with considerable fulness. The subjects discussed in this volume are so connected that it is not a little difficult to decide how they can be best arranged. I have determined in the first part to give, under the heads of the various animals and plants, a large body of facts, some of which may at first appear but little related to our subject, and to devote the latter part to general discussions. Whenever I have found it necessary to give numerous details, in support of any proposition or conclusion, small type has been used. The reader will, I think, find this plan a convenience, for, if he does not doubt the conclusion or care about the details, he can easily pass them over; yet I may be permitted to say that some of the discussions thus printed deserve attention, at least from the professed naturalist.