34 BULLETIlSr 1121, U. S. DEPARTMENT OF AGRICULTURE, 



unsettled, however, because those observed by different breeders 

 seemed irreconcilable. There were two main theories among breeders. 

 According to one, inbreeding has a specific detrimental effect, depend- 

 ing on its closeness and the length of time it is pursued, while the 

 introduction of outside blood has a specific stimulating effect. Ac- 

 cording to the other view, inbreeding merely concentrates and inten- 

 sifies the peculiarities of the given line, whether good, bad, or indif- 

 ferent. 



Neither of these theories was wholly satisfactory, the first because 

 it failed to account for the success of such men as Bakewell, the 

 Colling Brothers, Bates, Hewer, Cruickshank, and other noted 

 breeders who practiced inbreeding; the second because it did not 

 explain satisfactority the deterioration usually found on inbreeding 

 a stock which appeared to combine every element of vigor. 



PRE-MENDELIAN EXPERIMENTS. 



The first systematic experiments on the subject were made by 

 Darwin, who tested the effects of self-fertilization and crossing on a 

 large number of plants. His results were closely similar in almost 

 every respect to those which we have found on inbreeding guinea 

 pigs. There was a similar degeneration in size, fertility, and vitality 

 in most cases in which plants were used which are normally cross- 

 pollinating. This degeneration did not continue indefinitely. More- 

 over, in some lines of a given species there would be little if any 

 degeneration after many generations of selfing, while other lines of the 

 same species degenerated rapidly. Crossing within a selfed line had 

 no effect, but crosses between different lines resulted in increased 

 vigor. 



These results, the explanation of which seems clear enough to us 

 to-day, were in some respects a puzzle to Darwin. He admitted that 

 he was unable to formulate any complete explanation. He inclined 

 toward the view that the degree of difference in the composition of 

 the uniting germ cells has a specific stimulating effect. The follow- 

 ing quotation expresses this view: 



There are two other important conclusions which may be deduced from my observa- 

 tions; firstly, that the advantages of cross fertilization do not follow from some mysteri- 

 ous virtue in the mere union of two distinct individuals, but from such individuals 

 having been subject during previous generations to different conditions, or to their 

 having varied in the manner commonly called spontaneous, so that in either case 

 their sexual elements have been in some degrees differentiated. And secondly, that 

 the injury from self-fertilization follows from the want of such differentiation in the 

 sexual elements. These two propositions are fully established by my experiments. — • 

 (The effects of cross and self fertilization in the vegetable kingdom. London, 

 1876, p. 443.) 



No fault is to be found with Darwin's experiments. That he was 

 unable to formulate a thoroughgoing explanation of them was due 



