[1] "The Fitness of the Environment."
The essential characteristic of the adaptive reaction appears to be that the organism concerned responds to changing conditions without losing certain attributes of behavior by which we recognize organisms in general and by which that organism is recognized in particular. It exhibits stability in the midst of change; it retains its identity. But this stability, let us repeat, is the stability of a certain type of physico-chemical system, with respect to certain characters only, and exhibited under certain circumscribed conditions. In so far as the problem of adaptation is thus restricted in its application, it remains a question of standards, a taxonomic convenience, a problem of the organism by definition only, empty of fundamental significance.
It is to be expected that systems differing widely in composition and structure will differ in their responses to given conditions. This will be true whether the systems compared thus are organic, or inorganic, or representative of both groups. The compounds of carbon, of which living substance is so characteristically composed, exhibit properties and reactions that distinguish them at once in many respects from the compounds of lead or sulphur. They also differ widely among themselves; compare, in this connection, serum albumen, acetic acid, cane sugar, urea. No vitalistic factor is needed for the interpretation of divergencies of this kind. But there are many significant similarities between organisms and inorganic systems as well. These are so frequently overlooked that it will now be desirable to consider a few illustrative cases. For the sake of brevity, they have been selected as representative of but two types of adaptation commonly known under the names of ACCLIMATIZATION and REGULATION.
Let us first consider the case of organisms which become acclimatized by slow degrees to new conditions that, suddenly imposed, would produce fatal results. Hydra is an organism which becomes thus acclimatized finally to solutions of strychnine too strong to be endured at first. Outwardly it appears to suffer in the process no obvious modifications. Yet modifications of a physiological order take place, as is shown, first, by the necessary deliberation of the acclimatization, second, by the death of the organism if transferred abruptly back to its original environment.
In other forms the structural changes accompanying acclimatization may be far more conspicuous. For example, the aerial leaves of Limnophila heterophylla are dentate, while those grown under water are excessively divided. Again, the helmets and caudal spines of Hyalodaphnia vary greatly in length with the seasonal temperature.
In these and the large number of similar cases that might be cited, stability of the physiological system under changed conditions is only obtained by changes in the system itself which are often exhibited by striking structural modifications.
Compare with such phenomena of acclimatization the responses of sulphur, tin, liquid crystals and iron alloys to changes of temperature. The rhombic crystals that characterize sulphur at ordinary temperatures and pressures, give place to monoclinic crystals at 95.5 degrees C. Sulphur thus exists with two crystalline forms whose stability depends directly upon the temperature.
Similarly, tin exists under two stable forms, white and gray, the one above, the other below the transitional point, which is, in this case, 18 degrees C. At this temperature white tin is in a metastable condition, and transforms into the gray variety. The transformation goes on, then, at ordinary temperatures, but, fortunately for us as users of tin implements, very slowly. Its velocity can be increased, however, by lowering the temperature, on which, then, not only the transformation itself, but its rate depends.
In this connection may be mentioned cholesteryl acetate and benzoate and other substances which possess two crystalline phases, one of which is liquid, unlike other liquids, however, in being anisotropic. As in the preceding cases, these phases are expressions of equilibrium at different temperatures.
Especially instructive facts are afforded by the alloys of iron and carbon. Iron, or ferrite, exists under three forms: as alpha ferrite below 760 degrees, as beta ferrite between 760 degrees and 900 degrees, and as gamma ferrite above 900 degrees. Only the last is able to hold carbon in solid solution. The alloys of iron and carbon exist under several forms. Pearlite is a heterogeneous mixture containing 0.8 per cent. carbon. When heated to 670 degrees, it becomes homogeneous, an amount of carbon up to two per cent. dissolves in the iron, and hard steel or martensite is formed. In appearance, however, the two forms are so nearly identical as to be discriminated only by careful microscopical examination. Cementite is a definite compound of iron and carbon represented by the formula Fe<3 subscript>C.