HYBRIDS 137 



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 secret- 

 ing 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 

 incompatability 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 



