GROWTH. 314 GROWTH. ganisms develop with abnormal slowness in a salt solution. (4) Molar Agents. — Quiet facilitates growtli, and after a wound excessive growth takes place along the cut edge. (5) Light. — Light may retard or accelerate growth in different cases. Its effect is usually unfavorable to growth in young plants, and the growing tissue of vegetative plants is usually jirotected from the sun's rays. So, likewise, among animals the embryo is usually sheltered from the light by being carried in the iiuiternal body, covered by egg-shells, or placed in dark places. Acceleration of growth by light, on the other hand, takes place in certain aquatic algfe, especially diatoms, and in certain aquatic em- bryos (tadpoles, fish-embrj'os, and young pond- snails). In these processes of retardation and acceleration not all the component rays of white are equally effective. Thus, in general, the long- waved raj'S have no more efl'ect than darkness except in forming starch. It is the active ra,ys which retard the growth of aerial organisms, and it is the actinic rays which accelerate the growth of aquatic organisms. (0) Tcmpiraliirc. — Heat has a well-knoTi effect on the rate of growth. For every organism there is a temperature to which it is attuned — • at which growth takes plaoe faster than at any other. As we lower the temperature the rate of growth gi-advially and slowly diminishes; as we raise the temperature the rate of growth diminishes more rapidly. The temperature of maximum growth (called the optimum) is closely related to the temperature to which the organism is normally subjected. See Evolution. (7) Extent of the Medium of Volume. — Among the most remarkable factors determining the e.- tent of growth of an animal is the amount of space it has to move in. It is well known that small trout occur in small brooks and large ones in greater streams. Some years ago Karl Sem- per reared pond-snails in vessels containing different quantities of water. He found that there was a direct relation between the volume of the water and the length of the shell at the end of eight weeks. This experiment has since been repeated with care, and the result con- firmed. The explanation of this phenomenon is difficult. It may be that there is a self-adjust- ment of the size of the snail to the space at its command. In fact, in all the phenomena of growth, the jihenomena of adaptation are strik- ing. As to the external factors which influence growth, we are very ignorant. Morgan speaks of a few isolated cases. From experiments in cutting off the tail of a minnow, in which par- ticular regions are held in check while others grow at the maximum rate, the results demon- strate some sort of a formative influence in the new part. "An excellent example of an internal factor," Morgan says, "is found in the inter- relations of the parts of the organism to each other." A most important factor in the growth of organisms of the present day is that of heredity (q.v. ), for the existing forms of life are the descendants of innumeralile generations. Growth, development, and reproduction are now proceed- ing in grooves as it were, or along more or less definite paths, in accordance with long-establish- ed laws, and heredity undoubtedly affects the mechanism of growth. As Cope has well said: "In bathmism we see the resultant of innumer- able antecedent influences, which characterize the life of living beings." When a worm multiplies by budding or by self-division, the new part grows or begins to form in a zone near the end of the body. This zone is called the 'growing zone,' or the "zone of proliferation,' and here the nuclear substance is abundant in proportion to the enveloping pro- topla.sm. This zone is still more definite in the embryos of all arthropods. It is situated between the peiuiltimate and the last segment of the body, and from this zone the new segments arise and grow. It is also interesting to observe, as Packard states, that in striped or spotted cater- pillars, as well as in lizards, the change in the origin and shape of the markings appears to originate at what corresponds to the zone of formative cells, and then passes forward toward the head. Growth is related to longevity (q.v.). That men, says Minot, are larger than rabbits, because they grow for a longer jjeriod is a common obser- vation; but the facts tabulated by him show the curious result that the average daily increments in growth are nearly the same in man and rabbit. "On the other hand, rabbits attain a larger size than guinea-pigs, not because they grow longer, but because they grow much faster." The rate of growth in guinea-pigs and rabbits is not very different, as was to be expected in so nearly related animals ; but it is sufficient to produce a great inequality in the final size, because the effect of the difference is accumulated, like com- pound interest on capital. The rate in man (0.02 per cent.) is only J^ of the rabbit's (0.5 per cent.). This comparison renders it still more strikingly evident that the larger size of man depends on the greater duration of growth." The percentages given by Jtinot may, he says, bo called the specific ooefficicnts of growth. Together with the duration of growth, they de- termine the ultimate size of the organism. As a rule the larger an animal, the longer it lives, and this, adds Minot, confirms the idea that in large animals senescence proceeds more slowly than in smaller ones. See Sene.scence. Growth is related to great size. The question why the elephant is so much larger than the mouse, the whale than the porpoise, cannot be answered at present ; but from the foregoing statement the causes appear to be connected with nutrition and longevity, as well as the conditions of the environment. Why the moa (Dinomis) or the epiornis. whose legs were as thick as a man's hat, attained such a colossal size com- pared with the sparrow, is apparently due to its mode of life and its environment. Birds which have the full use of their wings, and are con- stantly on the wing, live faster than running birds, and are of small or moderate size. The ancestors of the nmning bird, as the ostrich and moa, were undoulitedly flying birds, with keeled sterna; but owing to the isolation of the moa on the New Zealand Islands, and to the absence of mammalian and other enemies, it is evident that by lack of exercise, by food being abundant in the soil, and by not being compelled to fly after their prey, they gradually increased in bulk and weight. Here we see a direct relation between the conditions of life and change of habits, and size. Turtles are slow livers, and are known to attain a verv great age.
