This power of resistance or immunity to fertilization by other species may be attained by several different methods. Amongst these are (1) a difference in the season of breeding or sexual ripening; (2) the production of secretions (whether by plant or by animal) which poison or paralyse the fertilizing sperms of allied and locally associated species, but are harmless to those of the secreting species; (3) the mechanical differences of size, etc., which prevent the fertilizing material of a strange species from gaining access to the egg-cells; (4) psychical activities (antipathies) in the case of animals or mere attraction and repulsion by odoriferous substances, which serve to repel a strange species, but are attractive to individuals of the same species; (5) finally, a chemical and physiological incompatibility between the sperms of one species and the germs of another (as distinct from the attraction or repulsion of the entire living individual), which, even when all other difficulties are absent or have been overcome, may be, and frequently is, present, so that the spermatozoon cannot penetrate the egg-cell even when resting upon it, but may be paralysed or repelled, and in any case is not guided and drawn into the aperture of the egg-covering, called the micropyle, or "little entry," so as to fuse with and fertilize the egg. The operation of these hindrances to hybrid fertilization and breeding have been ascertained in several different instances. It is not always possible, and certainly not easy to ascertain, which is at work in any and every case. But we can well conceive that one or other of these agencies have been developed and accentuated by survival of the fittest, so as to protect a species against fertilization by a neighbouring species, and thus to enable it to maintain its own "bundle of characteristics" free from the swamping effects of "mixture" (that is, "hybridization") with another species. It is also thus intelligible that an allied species from a distant land against which our native species and its closer ancestry–struggling for purity of race–have had no occasion or opportunity to develop a repelling protection–will have no such difficulty in effecting the fertilization of the native species as have those adjacent species against whose intrusions the latter is specifically moulded and selected by long generations of severe natural selection.