28 GENERAL PRINCIPLES OF ZOOLOGY. 



systematic distinctions as does the common systematic experience, 

 viz., the depreciation of constancy and the divergence of distin- 

 guishing characters ? 



B. The Intercrossing of Species. This field is as yet far from 

 being sufficiently investigated experimentally; yet some general 

 propositions can be set up: (1) that not a few so-called 'good 

 species' can be crossed with one another; (2) that in general the 

 difficulty of crossing increases, the more distant the systematic 

 relationship of the species used; (3) that these difficulties are by 

 no means directly proportional to the systematic divergence of the 

 species. The most favorable material for research is furnished by 

 those animals in which artificial fertilization can be carried out, 

 i.e., of which one can take the eggs and spermatozoa and mix 

 them independently of the will of the animals. Thus hybrids 

 have been obtained from species which belong to quite different 

 genera, while very often nearly-related species will not cross. 

 Among fishes we know hybrids of Abramis brama and Blicca 

 bjorkna, of Trutta solar (salmon) and Trutta far io (trout); among 

 sea-urchins the spermatozoa of Strongylocentrotus lividus fertilize 

 with great readiness the eggs of Echinus microtuberculatus, but 

 only rarely the eggs of Sphcerechinus granularis, which is nearer 

 in the system. It also happens that crossing in one direction 

 (male of A and female of B) is easily accomplished, but in the 

 other direction (male of B and female of A) it completely fails; as, 

 for example, the sperm of Strongylocentrotus lividus fertilizes well 

 the eggs of Echinus microtuberculatus, but, conversely, the sperm 

 of E. microtuberculatus does not fertilize the eggs of S. lividus. 

 Even better known is the fact that salmon eggs are fertilized by 

 trout sperm but not trout eggs by salmon sperm. Eggs have been 

 fertilized by sperm belonging, to different families, orders, and 

 possibly classes. Eggs of Pleuronectes platessa and Labrus rnpestris 

 by sperm of the cod (Gadus morrhua), frogs' eggs (Rana arvalis) 

 by sperm of two species of Triton, eggs of a starfish (Asterias 

 forbesi) by milt from a sea-urchin, Arbacia pustulosa (??). In 

 these extreme cases, it is true, the hybrids die during or at the 

 close of segmentation, before the embryo is outlined. 



In the case of animals where copulation is necessary the diffi- 

 culties of experimentation increase, since here often between males 

 and females of different species there exists an aversion which 

 prevents any union of the sexes. Yet in this case we know crosses 

 of different species; among the vertebrates crossing takes place, 

 e.g., between the horse and the ass; our domestic cattle and the 



