EXPERIMENTS IN HYBRIDIZATION AND TEMPERATURE. 341 



Nos. 4 and 7 of these nine crossings only produced a small num- 

 ber of larvffi, which were not successfully reared. From the 

 remaining seven crossings imagines were bred. One (No. 5) only 

 produced a few males, and two others (Nos. 3 and 6) only a few 

 females, the latter always having the organs of reproduction 

 undeveloped. No. 1 produced male imagines in numbers, but 

 only a very few crippled females. Three crossings (Nos. 2, 8, 

 and 9) produced males and females in normal proportions, but 

 only one of these crossings (No. 2) produced females with 

 the ovaries properly filled with eggs ; the females of the other 

 two crossings (8 and 9) possessed no germs, except a few females 

 of No 8, which quite exceptionally had a few crippled and mal- 

 formed ones. 



The fundamental law to be drawn from these experiments, as 

 indicative of the character of hybrids, is as follows : — 



Primary hybrids — that is, the result of crossing genuine 

 species drawn from nature — generally produce an individual (in 

 most cases an intermediate form between the two parent species) 

 which is relatively nearer to the phylogenetic older species than 

 the newer. The older species enforces its biological, morpho- 

 logical, and physiological characters on its hybrid offspring to a 

 greater degree than the younger. This law was proved in the 

 following manner : — 



The highly interesting fact was observed that the male of the 

 small emperor {S. pavonia) had very little influence on a brood of 

 eggs obtained by crossing it with a female S. spini, which is not 

 much bigger than the small emperor ; whereas the male S. 

 pavonia had a great influence on the brood of the large S. pyri, 

 which is about eleven times as big. Why does this same creature 

 {S. pavonia ^ ) so largely alter by crossing the brood of the giant 

 S. pyri, when it has so little influence on that of S. spini, which 

 is about as large as itself ? 



A comparison of the larval and pupal stages of the three 

 species — S. spini, pavonia, and pyr^i — showed in several regards 

 three different grades of protective resemblance respecting certain 

 inimical factors of nature. On this genealogical tree spini 

 always took the lowest place, pyri always the highest. On 

 account of the extraordinarily close relationship, and the great 

 similarity of the biological conditions of the three species, we 

 are forced to the conclusion that spini is the phylogenetic oldest ; 

 pavonia, younger ; and jj?/?-i the phylogenetic youngest form. 

 (Further particulars can be obtained : Standfuss, Handbuch, 

 1896, pp. 100-107.) Applying this conclusion to the results of 

 the crossing experiments : the oldest species {spini) was able to 

 cling to its characters more strongly than the younger pavonia, 

 and this latter again more strongly than the still younger pyri. 

 A long genetic past thus strengthens specific characters. 



ENTOM. DECEMBER, 1900: 2 G 



