The sensitive area which forms the basis of tropisms is as a rule developed not in the whole organism but only in certain segments of the body. Thus the eyes are located in the head. But when the action of one segment becomes overpowering the whole organism follows the segment. It has been customary among physiologists to speak of reflexes in such cases. Thus, e. g., the arms of the male frog develop a powerful positive stereotropism on their ventral surface during the spawning season. It would avoid confusion to realize that there is nothing gained in applying to this tropism the meaningless term “reflex”; it is better to call them tropisms since the organism as a whole is involved. If necessary we might speak of segmental tropisms. The act of seeking the female as well as that of cohabita­tion are in many cases combina­tions of chemotropism and stereotropism. The development of these tropisms depends upon the presence of certain specific substances in the body, a fact emphasized already in the case of helio­tropism. In case of the development of the segmental stereotropism of the male frog at the time of spawning it has been shown that it depends on an internal secre­tion from the testes.

It has been suggested by some authors that the tropistic reac­tions are determined by some feeling or emotion on the part of the organism. We have no means of judging the emotions of lower animals (except by “intui­tion”). The writer suggested in 1899 in his book on brain physi­ology that emotions may be determined by specific substances which also determine the tropistic reac­tion (as well as phenomena of organ forma­tion, although this latter phenomenon has nothing to do with the subject of instincts); and the excellent work of Cannon[246] has shown the rôle of adrenalin in the expression of fear. It is, therefore, both unwarranted and unnecessary to state that hypothetical emotions determine the tropistic reac­tions.

CHAPTER XI

THE INFLUENCE OF ENVIRONMENT

1. The term environ­ment in rela­tion to an organism may easily assume a mystic rôle if we assume that it can modify the organisms so that they become adapted to its peculiarities. Such ideas are difficult to comprehend from a physico­chemical viewpoint, according to which environ­ment cannot affect the living organism and non-living matter in essentially different ways. Of course we know that proteins will as a rule coagulate at temperatures far below the boiling point of water and that no life is conceivable for any length of time at temperatures above 100° C., but heat coagula­tion of proteins occurs as well in the test-tube as in the living organism. If we substitute for the indefinite term environ­ment the individual physical and chemical forces which constitute environ­ment it is possible to show that the influence of each of these forces upon the organism finds its expression in simple physico­chemical laws and that there is no need to introduce any other considera­tions.

We select for our discussion first the most influential of external condi­tions, namely temperature. The reader knows that there is a lower as well as an upper temperature limit for life. Setchell has ascertained that in hot springs whose temperature is 43° C., or above, no animals or green alga are found.[247] In hot springs whose temperature is above 43° he found only the Cyanophyceæ, whose structure is more closely related to that of the bacteria than to that of the algæ, inasmuch as they have neither definitely differentiated nuclei nor chromophores. The highest temperature at which Cyanophyceæ occurred was 63° C. Not all the Cyanophyceæ were able to stand temperatures above 43° C., but only a few species. The other Cyanophyceæ were found at a temperature below 40° C., and were no more able to stand higher temperatures than the real algæ or animals. The Cyanophyceæ of the hot springs were as a rule killed by a temperature of 73°. From this we must conclude that they contain proteins whose coagula­tion temperature lies above that of animals and green plants, and may be as high as 73°. Among the fungi many forms can resist a temperature above 43° or 45°; the spores can generally stand a higher temperature than the vegetative organs. Duclaux found that certain bacilli (Tyrothrix) found in cheese are killed in one minute at a temperature of from 80° to 90°; while for the spores of the same bacillus a temperature of from 105° to 120° was required.[248]

Duclaux has called atten­tion to a fact which is of importance for the investiga­tion of the upper temperature limit for the life of organisms. According to this author it is erroneous to speak of a definite temperature as a fatal one; instead we must speak of a deadly temperature zone. This is due to the fact that the length of time which an organism is exposed to a higher temperature is of importance. Duclaux quotes as an example a series of experi­ments by Christen on the spores of soil and hay bacilli. The spores were exposed to a stream of steam and the time determined which was required at the various temperatures to kill the spores.