When these facts are considered we have to admit that in the mating of two species which will not regularly and naturally breed together, there may be a limited action of the spermatic element which may stimulate the egg to development without contributing by fusion in the regular way to the actual substance of the young so produced, or only contributing an amount insufficient to produce a full and normal development of the hybrid young. Such cases not improbably sometimes occur in higher animals, though they have not been, as yet, shown to exist except in the experiments with sea-urchins' eggs and feather-star's sperm. In all animals and plants, but especially in domesticated and cultivated stocks or strains, varieties arise which, by natural or artificial separation, breed apart, and give rise to what are called "races." Such races in natural conditions may become species. Species are races or groups of individuals, which, by long estrangement (not necessarily local isolation) from the parent stock and by adaptation to special conditions of life, have become more or less "stable"–that is, permanent and unchanging in the conditions to which they have become adapted. They acquire by one device or another the habit of not breeding with the stock from which they originally diverged–a repugnance which may be overcome by human contrivance or by natural accident, but is, nevertheless, an effective and real quality. Distinct forms, which have not arrived at the stability and separation characteristic of species, are spoken of as "races," or "varieties." It is very generally the case that the "races" of one species can inter-breed freely with one another, and with the original stock, when it still exists. Comparatively little is known as to the behaviour of wild or naturally-produced "races." Practically all our views on the subject of "races" and their inter-breeding are derived from our observation of the immense number and range of "races" and "breeds" produced by man–as farmer, fancier, and horticulturist. It has been generally received as a rule, that the various races produced in the farm or garden by breeding from a species, will inter-breed freely, and produce offspring which are fertile. A special and important series of races, in which human purpose and voluntary selection necessarily have a leading part, are the races of man.