Most naturalists will agree, in all probability, with this conclusion of Darwin’s. The examples cited in the preceding pages have shown that there are several ways in which the organisms may respond to the environment. In some cases it appears to affect all the individuals in the same way; in other cases it appears to cause them to fluctuate in many directions; and in still other cases, without any recognizable change in the external conditions, new forms may suddenly appear, often of a perfectly definite type, that depart widely from the parent form. For the theory of evolution it is a point of the first importance to determine which of these modes of variation has supplied the basis for evolution. Moreover, we are here especially concerned with the question of how adaptive variations arise. Without attempting to decide for the present between these different kinds of variability, let us examine certain cases in which an immediate and adaptive response to the environment has been described as taking place. Responsive Changes in the Organism that adapt it to the New Environment There is some experimental evidence showing that sometimes organisms respond directly and adaptively to certain changes in the environment. Few as the facts are, they require very careful consideration in our present examination. The most striking, perhaps, is the acclimatization to different temperatures. It has been found that while few active organisms can withstand a temperature over 45 degrees C., and that for very many 40 degrees is a fatal point, yet, on the other hand, there are organisms that live in certain hot springs where the temperature is very high. Thus, to give a few examples, there are some of the lower plants, nostocs and protococcus forms, that live in the geysers of California at a temperature of 93 degrees C., or nearly that of boiling water. Leptothrix is found in the Carlsbad springs, that have a temperature of 44 to 54 degrees. Oscillaria have been found in the Yellowstone Park in water between 54 and 68 degrees, and in the hot springs in the Philippines at 71 degrees, and on Ischia at 85 degrees, and in Iceland at 98 degrees. It is probable from recent observations of Setchel that most of the temperatures are too high, since he finds that the water at the edge of hot springs is many degrees lower than that in the middle parts. The snail, _Physa acuta_, has been found in France living at a temperature of 35 to 36 degrees; another snail, Paludina, at Abano, Padua, at 50 degrees. Rotifers have been found at Carlsbad at 45 to 54 degrees; Anguillidæ at Ischia at 81 degrees; _Cypris balnearia_, a crustacean at Hammam-Meckhoutin, at 81 degrees; frogs at the baths of “Pise” at 38 degrees.