30 Mr Creighton, On the order in which the secreting [Feb. 26, 



A comparison of these two species with the other two will 

 reveal at once that the similitude is in this instance much less 

 masked. The Asilus crabroniformis is in bulk and width much 

 inferior to our native hornet. Although the fourth, fifth, sixth and 

 seventh segments of the abdomen are yellow, and so correspond 

 with the last segment of the abdomen of the hornet, they have no 

 maculation, as those of the hornet have. The velvety black of the 

 three basal segments of the abdomen in the fly detract from the 

 likeness also. Indeed, the dried and pinned-out insects are so 

 unlike that one wonders that they could have been thought to be 

 alike, and yet I can assure those who have not had the oppor- 

 tunity of observing them in the field, that when thus seen this fly 

 cannot fail to remind one of the Vespa crabro, after which it is 

 named. 



Here then we have two similitudes of different degrees, to 

 which many more might be taken in pairs from the same two 

 families, and these would represent many gradations. 



Now the gradation of an advantageous useful character, from a 

 case in which the protective similitude can scarcely be doubted by 

 any one to those in which it is barely recognizable, certainly seems 

 to indicate a transmutability of species. If fixity were the law, we 

 might expect perfection in a protective resemblance so far as is 

 consistent with the habits and functions of the two species. 

 Whereas, if all species be transmutable it will need long con- 

 tinuance of unchanged conditions to allow two utterly different 

 species to approximate one another even in aspect, and there will 

 be various stages in their approximation. In other words, where 

 progression and evolution is the law, imperfection is incidental to 

 perfection. If fixity be the law, anything short of absolute 

 perfection is an unexplained fault in nature. 



February 20, 1877. 

 Prof. Clerk Maxwell, President, in the chair. 



The following communication was made to the Society : 



Mr C. Creighton, On the order in luhich the secreting and 

 the conducting parts ■ of an acinous gland appear, in the indi- 

 vidual development, and in the succession of animals. 



The early appearance of the ducts in the embryonic develop- 

 ment of acinous glands is the fact in their development that has 



