202 _ LECTURES ON BIOLOGY 



It might be asked here whether new species may not also 

 be produced by crossing different species. This question is 

 justified, but as I shall have to refer later on, when dealing 

 with the theory of heredity, to the famous crossing-experiments 

 of Mendel, I need here allude to it only very briefly. 



As long as the dogma of the constancy of the species was 

 omnipotent the possibility of a crossing of species was flatly 

 denied. This denial is all the less comprehensible as the 

 bastards of the ass and the horse were known in Egypt, Persia 

 and India thousands of years ago. It is, however, possible that 

 as both mule and hinny are sterile this instance was not regarded 

 as possessing any value. But to-day we know a large number 

 of results of crossing not only different species, but even different 

 genera. I need only mention the bastards of sheep and goat, 

 dog and wolf, hare and rabbit, canary and different finches, 

 salmon and trout. It is true that in most cases the bastards 

 possess degenerate sex-organs and are sterile, but we know, 

 nevertheless, already quite a large number of crosses which are 

 capable of reproduction. The bastards of rabbits and hares 

 have remained fertile through many generations, and bastards 

 of the domestic goat and the steinbok, of different species of 

 butterfly, etc., have produced several generations of descendants. 



It is remarkable that it is very easy to cross the male of one 

 species with the female of another, but not vice versa. Thus 

 the egg of salmon may be fertilized with the seed of trout and 

 developed, but every attempt to cause the development of trout- 

 ova with the spermatozoa of salmon has ended in failure. 



Many interesting results were derived from experiments 

 which Standfusz made with butterflies. Here it appeared that 

 the products of crossing generally followed in appearance the 

 phylogenetically earlier form. If, for instance, we cross the 

 female of the Poplar Hawk-moth (Smerinthus populi} with a 

 male of the Eyed Hawk-moth (Smerinthus ocellatus'), the 

 descendants of the two species resemble more the phylo- 

 genetically older S. populi than S. ocellatus. The same result 

 was obtained in crossing a male Dilina tilice with S. ocellatus. 

 In this case, too, the characters of the phylogenetically earlier 

 kind predominated in the descendants. 



It seems not open to any doubt that by crossing we can obtain 



