—235 — 



which can only be made satisfactorily at night with the assistance of a 

 confederate. If Mr. Hulst has any valid reasons for not coming to the 

 sa*ne conclusions, he will better advance our knowledge by giving his 

 reasons than by the mere expression of an opinion without evidence that 

 he is in any way justified in it. Until he does the presumption is that 

 he has carelessly observed or has even confounded Pronuba and Prodoxus 

 as others have done before him. 



Finally, I commend to Mr. Hulst a more careful study of my writ- 

 ings upon this subject and especially those in the Proceedings of the 

 American Association for the Advancement of Science for i88oand 1882, 

 and when he has more critically observed the insect in the plant he will cor- 

 roborate all that I have written in the way of facts and observations and 

 agree with me that they lully justify the opinions expressed, which may 

 be summed up thus: 



I. Yucca by the very nature of its flowers is normally incapable of 

 self-fructification. This is essentially true of the dehiscent species which 

 occur in the Northern States. 



II. Pronuba yuccasella is modified in the female so as to enable her 

 to gather the pollen and to fructify the fruit. Careful observation ol the 

 insect itself proves that function is here, as elsewhere, rightlv correlated 

 with structure and that not a solitary case of Yucca fructification bv an- 

 other insect has ever yet been witnessed by any one. 



III. The only exception to the fruiting of Yucca without the help 

 of Pronuba is in A. aloifolia, ami while I have admitted the possibility of 

 exceptional pollination by other insects, the fact remains that none have 

 been observed to have the power. I have shown that self-fertilization 

 may take place with this species and that its exceptional fruiting where 

 Pronuba is out of the question is more likely due to such fertilization 

 than to the agency of other insects; because, from personal observations, 

 both published and partly unpublished, over the whole region in which 

 the plant indigenously grows, I know trie fruit to be abundant where 

 Pronuba occurs and that it then always hears evidence of her presence* 

 whereas where she does not occur the fruit is scant and very excep- 

 tional. 



IV. That a certain variable proportion of Yucca capsules develop 

 without trace of the Pronuba larvae is explained bv the fact that pollina- 

 tion, as 1 have shown; often takes place without successful oviposition; 

 or that successful hatching of the eggs or development of the larvae ma} 

 be prevented from various causes. That pollination is rare without 

 oviposition is proved by my direct observations of the acts and bv the 

 fact that comparatively few perfect capsules (or those which do not show 

 the evidence of oviposition) are to be found even where no larve" have 



