FERTILITY OF PARENT SPECIES AND HYBRIDS. 87 



concentrated the salt solution on the slide. The results showed that 

 a careful microscopic test, at the age of 5 months or over, is a very 

 reliable index of sterilty or fertility. 



In order to test a male by breeding, it is essential that he should be 

 healthy, and kept with vigorous adult females for a number of months. 

 Even then a male may be potentially fertile, but fail to impregnate a 

 female because of sluggishness or other external causes having no 

 obvious relation to the mere presence or absence of motile sperm. The 

 ideal test of fertility is the combination of a breeding and a microscopic 

 test. There were, in all, 50 males tested by breeding alone, and 102 

 males tested in both ways. Whenever the breeding test was used a 

 male was given every opportunity to demonstrate his fertility. The 

 unreliability of a simple breeding test, however, was evident to me 

 during the early part of the experiment, for a few males having an 

 abundance of motile sperm faUed to impregnate females, although 

 continually with these for many months. Two such males were about 

 to be given up as practically sterile after a breeding test of almost a 

 year; but on deciding to continue the test I was greatly surprised and 

 repaid by several litters from them. One of these two (c?375) did not 

 impregnate a female until after 18 months of continued breeding. I 

 suspect that some fertile hybrid males were not always as successful 

 breeders as normal guinea-pigs, even though it was absolutely impos- 

 sible to detect any difference in the abundance or character of their 

 spermatozoa. 



A total of 483 males was tested by one or both tests. The indi- 

 viduals ranged from the Fi through the Fg generation, most individuals 

 (329) belonging to the F3, F4, and F5 generations. The results are put 

 in tabular form as far as possible and recorded in tables 73 to 77. 



Table 72 shows how many hybrid males in each generation were 

 tested by either one or both methods. 



THE RESULTS OF THE SIMPLE BREEDING TESTS ALONE. 



About one-tenth of all the hybrid males were tested by a simple 

 breeding test. They ranged from the J wild to the ^V "^Id, a total of 

 50 individuals (see table 73). The breeding test was thorough and 

 there is no doubt that each of them, except one ^V wild male (cf 305), 

 was sterile for all practical breeding purposes. To be sure, some of 

 them may have had immotile sperm or even some motile sperm, but 

 they failed to impregnate any females as a normal guinea-pig would 

 have done under similar circumstances. We have no knowledge of 

 their germ cells. In the Ught of the other tests, these breeding tests 

 became more significant. 



The reason that so many hybrid males of the early generations were 

 not tested microscopically was because the animals were scarce and 

 valuable and it was feared that an operation upon the epididymis might 



