Stability and Real Atavism 175 



hitherto observed, seed-reversions must be said 

 to be extremely rare. Or rather, cases which 

 are sufficiently certain to be relied upon, are 

 very rare, and perhaps wholly lacking. Most 

 of the instances, recorded by various writers, 

 are open to question. Doubts exist as to the 

 purity of the seeds and the possibility of some 

 unobserved cross disturbing the results. 



In the next lecture we shall deal in general 

 with the ordinary causes and results of such 

 crosses. We may then see that they are so 

 common and occur so regularly under ordinary 

 circumstances that we can never rely on the 

 absolute purity of any seeds, if the impossibility 

 of an occasional cross has not been wholly ex- 

 cluded, either by the circumstances themselves, 

 or by experimental precautions taken during 

 the flowering period. 



For these reasons cases of atavism given 

 without recording the circumstances, or the pre- 

 cautions that guarantee the purity of the fertili- 

 zation, should always be disregarded. And 

 moreover another proof should always be de- 

 manded. The parent which yielded the seeds 

 might be itself a hybrid, and liable to reversions 

 by the ordinary laws of the splitting up of hy- 

 brids. Such cases should likewise be discarded, 

 since they bring in confusing elements. If we 

 review the long list of recorded cases by these 



