1917] Fossil Ifisects 3 



exhibited a number, of figures of hymenopterous wings, and 

 offered some opinions concerning the evolution of the venation. 

 In the discussion which followed, the criticism was made that 

 all the species concerned were living ones, that obviously they . 

 could not be thought of as ancestral to one another, and conse- 

 quently any attempt to see in them a true evolutionary series 

 must be futile. This sounded reasonable, but it did not take 

 into account the fact that while species may all be recent 

 genera of insects are old, and of extremely different antiquity. 

 This is one of the lessons we have learned from the study of 

 fossil insects, and it teaches us that the existing insect fauna is 

 extremely rich in ancient types, which do really illustrate 

 evolutionary sequence. The reason for this is rather obvious. 

 The stream of insect life branches in a complex manner and 

 owing to the enormous diversity of possible adaptations, 

 resulting from the diversity of physical conditions, of food and 

 of enemies, very many of the products of evolution have been 

 preserved without important modification. This is especially 

 striking when we regard characters rather than species, and 

 observe differences in the minute structure of the tegmina of 

 Palaeozoic cockroaches, corresponding with similar differences 

 to be found in their living representatives. Just as the infinite 

 variety of higher animal life has been built up from a scarcely 

 altered fundamental series of tissues, so families, genera and 

 species have arisen not so much from entirely new developments, 

 as from the shuffling of ancient characteristics. There is, of 

 course, no doubt that definite progressive evolution has taken 

 place among the insects just as among the vertebrates; thus 

 the greatly modified mouth-parts of bees and butterflies, 

 adapted for sucking the nectar of flowers, certainly came into 

 existence after the Palseozoic, and when plants with suitable 

 corollas had developed or were developing. 



There is no doubt that the Mesozoic, the period of the rise 

 of the higher plants, saw a remarkable development of insect 

 life, concerning which we know too little, owing to the relative 

 scarcity of fossils. It does not appear, however, that there is 

 much if any innate tendency to progress, without reference to 

 changing conditions. During the Tertiary epoch there seems to 

 have been little forward evolution, and in the north temperate 

 regions we may detect a very perceptible contraction and 

 impoverishment of the fauna since the Miocene. In the absence 



