( Ixxi ) 



Palc-eodictyoptera, which gradually assumed hard elytra : but 

 we have absolutely nothing in the geological record to support 

 this, nor are there any species which can be considered 

 transitional between this and other orders : all we know is 

 that the Coleoptei-a not only appeared, but appeared in vast 

 numbers and as highly differentiated as at the present time 

 during the age of the great Saurian reptiles, before the great 

 mass of our existing genera, and even families, of fish and 

 reptiles had come into existence.* It is this persistence of type 

 among the Insecta, coupled with the enormous differentiation of 

 species and varieties, that is so striking : with regard to the per- 

 sistence of type the explanation is to be found, perhaps, in the 

 fact that the various orders became adapted at an early 

 period to their special modes of life, and that therefore there 

 has been, in the main, no need for any structural alteration, 

 although external differences are still being brought about, 

 partly by discontinuous variation, and partly by natural 

 selection. I know that many people are entirely at variance 

 with any theory of discontinuous variation, but I think that 

 both in Coleoptera and Lepidoptera we come across sports 

 which have, apparently, more or less suddenly broken off from 

 the type and are, practically, new species. The variety of 

 species, of course, in both Lepidoptera and Coleoptera is 

 inBnite. In a letter I received recently from Dr. A. K. 

 Wallace, after speaking of the enormous differentiation of 

 families, genera, and species, the majority of which are quite 

 well defined, he proceeds as follows : — " Again, the enormous 

 variety of species in many of the genera, varying every 100 

 miles or so in the monotonous Amazon plain, in every valley 

 of the Andes, and in every island of the Malay Archipelago, 

 shows how variable and adaptable they are under the slightest 

 diversity of conditions. Taking all these facts into consider- 

 ation, and always remembering the tremendous severity of the 

 weeding-out process, so that of the hundreds of larvte and 

 imagos produced by one female butterfly, moth or beetle, only 

 two on the average survive to breed and replace the two 

 parents, and there seems to be no difficulty whatever in the 



* "The Geographical Distribution of Animals," A. 11. Wallace, vol. i, 

 p. 176, 1876. 



