"When we revolve in our minds, first, the great changes which we see naturally produced in animals after their nativity.... Secondly, when we think over the great changes introduced into various animals by artificial or accidental cultivation.... Thirdly, when we enumerate the great changes produced in the species of animals before their nativity.... Fourthly, when we revolve in our minds the great similarity of structure which obtains in all the warm-blooded animals.... Fifthly, from their first rudiment or primordium to the termination of their lives, all animals undergo perpetual transformations, which are in part produced by their own exertions;... and many of these acquired forms or propensities are transmitted to their posterity.... A great want of one part of the animal world has consisted in the desire of the exclusive possession of the female; and these have acquired weapons to combat each other for this purpose.... The final cause of this contest amongst the males seems to be that the strongest and most active animal should propagate the species, which should thence become improved. Another great want consists in the means of procuring food, which has diversified the forms of all species of animals.... All which seem to have been gradually produced during many generations by the perpetual endeavour of the creatures to supply the want of food, and to have been delivered to their posterity with constant improvement of them for the purpose required.... The third great want among animals is that of security, which seems much to have diversified the forms of their bodies and the colour of them.... The contrivances for the purposes of security extend even to vegetables.... Would it be too bold to imagine that in the great length of time since the earth began to exist ... all warm-blooded animals have arisen from one living filament, which the Great First Cause endued with animality;... possessing the faculty of continuing to improve by its own inherent activity, and of delivering down those improvements by generation to its posterity, world without end!"] [7: In this study Darwin came into communication, as early as 1839, with the Hon. and Rev. W. Herbert, afterwards Dean of Manchester, and received from him a personal account of his experiments on hybrids. It was Herbert who, as early as 1822, in the fourth volume of the "Horticultural Transactions," and in his work on the Amaryllidaceæ, 1837, declared that horticultural experiments have established, beyond the possibility of refutation, that botanical species are only "a higher and more permanent class of varieties." He extended the same view to animals, and believed that single species of each genus were originally created in a highly plastic condition, and that these have produced, chiefly by intercrossing, but also by variation, all our existing species.]