VARIABILITY, POLYMORPHISM, HEREDITY Like all other organisms the Protozoa can be profoundly modi fied by various external conditions such as temperature, nutrition and state of the environment. Scarcity of food supply often leads to dwarfing, a rich supply to relatively giant growth. Some proto zoans are so dependent on eternal factors, so malleable, that they may be altered beyond recognition. For example some Amoebae that live in half-moist surroundings by transference to water come to develop flagella and turn into flagellates (fig. 15). In view of this mutability it was earlier hoped—when the protozoans were regarded just as primitive organisms—that changes from one species to another might be observed directly. This hope has not been fulfilled.
In other words, the inheritance of acquired characters is as little proved in the Protozoa as in the higher animals and plants. Cer tainly characters "acquired" by a protozoan under the influence of special external conditions reappear in the offspring. But this reappearance does not last very long.
Modifications of this kind are called "enduring modifications," in contrast to genuine hereditary variations (so-called mutations) which have been observed in some forms and remain constant through any number of divisions and fertilizations.
We have already pointed out in the introduction that the dif ferent classes and orders of the Protozoa are not related to one another as those of the vertebrates are ; that on the contrary a vast number of groups have been united under the "phylum" Protozoa for the sake of convenience. On this account it is useless to speak of the "phylogeny" of the Protozoa as a whole. In a few groups only is it possible to distinguish more primitive from more highly developed forms and make a kind of genealogical tree. An example of such a developmental series has already been touched upon. In the Volvocineae it can be clearly seen how cell-colonies have arisen from single-celled types and how a division of labour has followed.
The protoplasm of all living creatures contains water and it can only survive dry conditions so long as it is in some way protected from drought. The vegetative stages of nearly all Protozoa being unprotected they can only live in a moist medium; their cysts on the other hand can remain dried up for a long time.
We find Protozoa therefore in all kinds of fresh waters, as well as in the sea. They are also found free in moist earth and moss, and as parasites they occur in the body-fluids (blood, intestines, etc.) and tissues of animals, occasionally even of plants. Their cysts can be found wherever they are carried by wind and by other organisms.
Not only is the distribution of the Protozoa strictly limited by water-content but it is further limited by temperature and salt content. Most free-living, i.e., non-parasitic, Protozoa will scarcely stand temperatures of more than 3o° C (86° F) and temperatures of above 45° C (1 13° F) or below zero C (32° F) are fatal to most species in the vegetative condition. In hot springs, however, infusorians have been found at temperatures of 64° C (148° F) and freezing has been resisted by some spe cies. In the same way too strong solutions of salts such as those in the Algerian salt lakes render protozoan life impossible.
Naturally one of the essentials of life is the possibility of getting food; this requirement is met in each of the habitats that we have mentioned for one species or an other. It is the most important factor, apart from temperature, that exercises a decisive influence in determining the life-cycles of protozoans which go into a period of winter rest in the encysted condition.
Every protozoan is more or less strictly adapted to its natural environmental conditions. Thus it may find life altogether impos sible in conditions varying a little from the normal, although these may be the natural ones for another species. For example, a freshwater protozoan cannot exist in the sea, nor a blood-parasite in fresh water. Yet there are Protozoa that are very adaptable in these respects; some species normally living in fresh water can live for a considerable time in the intestines of animals or in the dissolving fluids secreted by carnivorous plants; intestinal protozoans of some animals can live on in the intestines of others that have eaten their first host.