the tail gets relatively longer; and even when the total growth of the rat is prevented by underfeeding, the form continues to alter so that this increasing length of the tail is still manifest54.
| Full-fed Rats. | ||||
|---|---|---|---|---|
| Age in weeks | Length of body (mm.) | Length of tail (mm.) | Total length | % of tail |
| 0 | 48ôñ7 | 16ôñ9 | 65ôñ6 | 25ôñ8 |
| 1 | 64ôñ5 | 29ôñ4 | 93ôñ9 | 31ôñ3 |
| 3 | 90ôñ4 | 59ôñ1 | 149ôñ5 | 39ôñ5 |
| 6 | 128ôñ0 | 110ôñ0 | 238ôñ0 | 46ôñ2 |
| 10 | 173ôñ0 | 150ôñ0 | 323ôñ0 | 46ôñ4 |
| Underfed Rats. | ||||
| 6 | 98ôñ0 | 72ôñ3 | 170ôñ3 | 42ôñ5 |
| 10 | 99ôñ6 | 83ôñ9 | 183ôñ5 | 45ôñ7 |
Again as physiologists have long been aware, there is a marked difference in the variation of weight of the different organs, according to whether the animalãs total weight remain constant, or be caused to diminish by actual starvation; and further striking differences appear when the diet is not only scanty, but ill-balanced. But these phenomena of abnormal growth, however interesting from the physiological view, are of little practical importance to the morphologist.
The effect of temperature .
The rates of growth which we have hitherto dealt with are based on special investigations, conducted under particular local conditions. For instance, Queteletãs data, so far as we have used them to illustrate the rate of growth in man, are drawn from his study of the population of Belgium. But apart from that ãfortuitousã individual variation which we have already considered, it is obvious that the normal rate of growth will be found to vary, in man and in other animals, just as the average stature varies, in different localities, and in different ãraces.ã This phenomenon is a very complex one, and is doubtless a resultant of many undefined contributory causes; but we at least gain something in regard to it, when we discover that the rate of growth is directly affected by temperature, and probably by other physical {108} conditions. Rûˋaumur was the first to shew, and the observation was repeated by Bonnet55, that the rate of growth or development of the chick was dependent on temperature, being retarded at temperatures below and somewhat accelerated at temperatures above the normal temperature of incubation, that is to say the temperature of the sitting hen. In the case of plants the fact that growth is greatly affected by temperature is a matter of familiar knowledge; the subject was first carefully studied by Alphonse De Candolle,