HISTORY OF ZOOLOGY 21 



in other cases sharply circumscribed groups of forms, or races, can be 

 distinguished 'W'ithin the same species. In the race, tlie peculiar character- 

 istics are inherited from generation to generation with the same constancy 

 as in good species. This is shown in man, and in many pure races of 

 domesticated animals. 



Physiological Characters. — A. Crossing of Species and Varieties. — • 

 A critical examination leads to the conclusion that morphology is indeed 

 useful for grouping animals into species and varieties, but it leaves us in 

 the lurch when called upon to show the disinctions jjetween a species and 

 a variety. Therefore there remains open to the systematist only one 

 resource, i.e., to summon physiology to his aid. This has disclosed 

 considerable distinctions in reproduction. We should expect a priori 

 that the individuals of different species would not reproduce with each 

 other; on the other hand the individuals of one and the same species, even 

 though of different varieties or races, should be entirely fertile. One 

 must beware of arguing in a circle in proof of these two propositions; 

 so the question must read: does physiological experiment lead to the 

 same distinctions as does the common systematic experience? 



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

 sufficiently investigated; yet some general propositions can be set up: 

 (i) 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. 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 Imowhyhrids oiAbramisbrama a,nd Blicca hjorkna, 

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

 spermatozoa of Slrongylocentroius lividus fertilize with great readiness the 

 eggs of Echinus microtuberculatus, but only rarely the eggs oi Sphcerechinifs 

 gramdaris, 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 *vell the eggs 

 of Echinus microtuberctdatus , but, conversely, the sperm of E. microtuber- 

 culatus does not fertilize the eggs of 5. lividus; 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 rupestris by sperm of 

 the cod, frogs' eggs by sperm of two species of Triton, eggs of a starfish 

 {Asterias forbesii) by milt from a sea-urchin, Arbacia pusttdosa. In 



