inclined to regard these as indicating widely different periods. Here the facts of present distribution serve to make us hesitate. Different altitudes, different soils, different conditions of moisture and so forth, produce to-day very distinct sets of animals and plants, even in the same immediate region. Or, if we are dealing with marine forms, a littoral and a deep-sea fauna of precisely the same age would be very different; indeed it is doubtless not an exaggeration to say that the present shallow-water fauna of the Atlantic coast resembles the shallow-water fauna of Middle Tertiary times much more than the deep-sea fauna of to-day. Considerations of this sort have led careful paleontologists to attempt to estimate the climatic and other conditions surrounding the subjects of their investigations; thus, for instance, Dr. Matthew, in discussing the Tertiary mammals of northeastern Colorado, concludes that one series represents a plains or prairie fauna, the other a forest one. These are not of the same age, but the difference between them is clearly to be attributed to environmental conditions as well as the lapse of time.
Thus in the course of our enquiry we come back to the modern biota, and find it necessary to ascertain as accurately as possible what conditions permit the existence and migration of the several species. Neither the blue flax nor the snail exists everywhere within the region which we said, in general terms, that they occupied. The flax occurs at various altitudes, up to 10,000 feet, but always in more or less open places, in dry or at least not very moist soil. The snail also lives at different altitudes, but in moist places under vegetation. Thus, although when plotted on a map the ranges of the two would appear to largely coincide, it is probable that they never, or almost never, actually exist together. While spreading over enormous areas, they have picked their way, as it were, from one suitable spot to another, showing thereby how closely they are dependent upon a particular set of conditions. In a general way, mountains may be said to favor the spread of both, and for either the desert is an impassable barrier.
When we have ascertained the necessary conditions of moisture, heat, light, etc., we have not nearly solved the problem. Very important, in nearly every case, is the living environment. In the case of the flax, we know that it is very injuriously affected by an orange rust (Uredo lini (Pers.) Schum.), which extends practically throughout its range. This rust infests not only the blue flax, but other species as well, including some of the yellow ones. Consequently, when two geographical groups of flax plants meet, whether they are of the same species or diverse, there is always a possibility that one will convey the rust to the other, supposing that they are not both already infested. The same sort of thing is true of diseases of animals, as many races of men have found to their destruction, upon mingling with the white