2s GENERAL PllINClPLES OF ZOOLOGY. 



systematic distinctions as does the common systematic experience, 

 viz., the depreciation of con>stancy and the divergence of distin- 

 guishing characters ? 



r>. llie Intercrossing of Species. — This lield is as yet far from 

 being sufficiently investigated experimentally; yet some general 

 propositions can be set np: (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 ca7i be carried out, 

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

 them independently of the will of the animals. Tlius 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 Abratnis Irama and Blicca 

 hjbrlcna, of Trutta salar (salmon) and Trutta fario (trout); among 

 sea-iirchins the spermatozoa of Strongylocentrotus lividus fertilize 

 with great readiness the eggs of Echinus microtuierculatus, but 

 only rarely the eggs of SphcBrechinus 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 exam^Dle, the sperm of Strongyloccntrof ns lividus fertilizes well 

 the eggs of Ecldnus microtuierculatus, but, conversely, the sperm 

 of E. microticierculatus 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 Pleuronectcs platessa and Labrus rupestris 

 by sperm of the cod {Gadus morrliua), frogs' eggs {Rana arritlis) 

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

 forbesi) by milt from a sea-urchin, Arhacio 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., Ijctween the liorse and the ass; (Uir domestic cattle and the 



