Although in comparison with the hosts of living forms, researches hitherto made have resulted in the insignificantly small number of about 10,000 species of fossil insects, yet these few afford us a glimpse into the insect life of past ages. Such a collection of extinct species, moreover, much exceeds in numbers the recent forms in most university and private collections, which have become the basis of so many bold hypotheses. We can thus see or at least have some idea how in the course of millions of years the present mighty tree has grown up from so small and tender a plant.
Of the fossil insects thus far obtained, the larger part have come from that important period immediately preceding the age of man. This is designated the Tertiary period, or the age of mammals. Those insect remains preserved in fossil gum (Baltic amber), like artistic and permanent microscopic preparations, are indeed well known, and of these many thousand specimens have been accumulated in museums. On the other hand, less noted, but not less numerous, are the wonderful impressions found in many places in laminated shales, as in Œningen (Baden), Radoboj (Croatia), in Italy, on the Rhine, in Provence, in North America, etc. These are to be likened to nature's own printing and provide us with an atlas of the Tertiary fauna in which we find very many species that can scarcely be distinguished from those living to-day. With the exception of bird-lice, lice and fleas, all the principal existing groups of insect throngs are represented in Tertiary time, but the remarkable bizarre forms which especially delight our eyes to-day were much fewer in number then than now. Thus very few large butterflies and no striking types of beetles, such as we are accustomed to see in all shop-windows of the dealers, have been discovered.
Even though the character of the Tertiary fauna in general did not vary essentially from that now in existence, still the distribution of forms over the earth must have been far different. For instance, in Germany we find elements that now are met with only in tropical lands, from which follows many a conclusion as to the variations of climate and of the plant world. Moreover, the numerical distribution of species in kindred groups was likewise not the same as that at present in force, since among the Tertiary Hymenoptera a much smaller percentage of bees is found, among the Diptera there are more gnats than flies, among the Orthoptera far more grasshoppers than locusts, and only very few walking-sticks, etc.
Further, when it is stated that in the Tertiary period no single type of insect has been hitherto identified which does not still exist, and that therefore the numerous amber preparations and the impressions so beautifully preserved are as yet capable of giving no direct answer to our question, we must then turn to the next older period,