FERTILITY OF PARENT SPECIES AND HYBRIDS. 95 



Table A. 



Hybrid 

 generation. 



F2 

 F3 

 F4 

 F„ 



With 1 factor. 



. 50 fertile. 

 . 75 fertile. 

 . 875 fertile 

 2n-i_i 



2n- 



fertile. 



With 2 factors. 



.25 fertile. 

 .5625 fertile 

 .7656 fertile 

 2"-i-1 n 

 . on-i J 



fertile. 



With 3 factors. 1 With p factors. 



(.50)' fertile. 

 (.75)3 fertile. 

 (.875)3 fertile. 

 21-' -1 



(.50)P fertile. 



(. 75) P fertile. 

 (.875)P fertile. 

 211-1-1 ^p, 



(^^9^) tatUe.| (--:^^)''fertUe 



From these series we may say that in any given generation, F„, in 

 which the degree of wildness is ^n, the number of fertile males should 



be P" nZ ) ^ where n equals the number of the hybrid generation 



and p equals the number of factors. In actual breeding experiments 

 the chances of error would be great. To realize such a series of segre- 

 gates, the different classes of females of each generation would also have 

 to occur in approximately the expected proportions in order to give the 

 expected percentage of ultimate recessive males in the next generation. 

 This could only be accomplished by raising very large numbers. 



It is quite impossible to determine from our data whether or not 

 the percentage of fertile males in each generation corresponds in any 

 measure to a theoretical percentage which is based on a definite number 

 of factors; because, as tables 72 to 75 show, not all males with many 

 motile sperm could be tested also by breeding. Furthermore, it is 

 shown in table 76 that at least 10 per cent of the males whose micro- 

 scopic test gave every promise of being fertile were actually sterile 

 after a rigid breeding test. We may feel more confident of the propor- 

 tions with many motile sperm than of the proportions really fertile. 

 If we examine the percentage of males in each hybrid generation, the 

 contents of whose epididymis could not be distinguished from that of 

 a normal guinea-pig, we find (see table 75) the following series of per- 

 centages from the Fi to the F7 inclusive : 



0.00 0.00 14.29 33.33 60.67 69.39 73.33 



If we take the percentage of males with many motile sperm in the total 

 tested by all methods the series is about the same: 



0.00 0.00 9.46 32.38 60.67 69.39 73.33 



This latter series of percentages imputes that all males sterile in a 

 thorough breeding test alone did not have many motile sperm. From 

 table 76 we see that this is not completely true in about one-tenth 

 of the cases. The first series is probably more accurate, as it is the 

 percentage of males with many motile sperm in the total of microscopic 



