34 GENERAL PRINCIPLES OF ZOOLOGY. 



Trutta salar (salmon) and Trutta fario (trout) ; among sea- 

 urchins the spermatoza of Strongylocentrotus lividns fer- 

 tilize with great readiness the eggs of EcJiinus microtnbcr- 

 culatus, but only extremely seldom the eggs of Splurrc- 

 chinus gramtlaris, which stands closer to it 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 fer- 

 tilizes well the eggs of EcJiinus microtuberculatus, but, con- 

 versely, the sperm of E. microtuberculatus does not fertilize 

 the eggs of 5. lividns. 



In the case of animals where copulation is necessary 

 the difficulties 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 zebu; 

 ibex (or wild buck) and she-goat ; sheep and goats ; dog 

 and jackal ; dog and wolf ; hare and rabbit (Lepns Dar- 

 ivini] ; etc. ; among birds, between different species of 

 finches and of grouse; mallard (Anas bosclias) and the 

 pintail duck (Dafila acutd); the European goose and the 

 Chinese goose (A user ferns and A. cygnoidcs). 



C. Fertility of Hybrids and Mongrels. Since many hy- 

 brids, as the mule, have been known for thousands of years, 

 the criterion is, as it were, pushed back one stage ; if the 

 infertility in cases of crosses in many species is not im- 

 mediately noticeable, yet it may be apparent in the prod- 

 ucts of the cross. While the products of the crossing of 

 varieties, the "mongrels," always have a normal, often an 

 increased, fertility, the products of the crossing of species, 

 the hybrids, should always be sterile. But even this is a 

 rule, not a law. The mule (which only very rarely repro- 

 duces) and many other hybrids are indeed sterile, but 

 there are not a few exceptions, although the number of 

 experiments in reference to this point is very small. 



