34 BULLETIN 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 
