402 THE WONDER OF LIFE 



dependence on nutrition and on external agencies, growth 

 is often punctuated in some detail. Every one is familiar 

 with the annual rings of growth seen on the cross-section 

 of a tree seen so clearly because there is an alternation 

 of summer wood and autumn wood differing in texture. 

 The more prominent lines on the shell of the freshwater 

 mussel indicate years and the weaker lines between these 

 indicate minor periodicities. But the finest registering 

 is seen on the scales, in the ear-stones, and even in some 

 of the bones of many fishes. 



Besides the periodicities of growth which can be reason- 

 ably correlated with external periodicities, such as those of 

 the seasons, there are others of a more recondite nature, 

 such as phases of quick growth and slow growth, that alter- 

 nate in the development of some animals, as Fischel has 

 shown, for instance, in the development of the duck. It is 

 probable that these differences of rate are connected with 

 the periodic liberation of internal secretions within the 

 growing organism. 



The rate of growth has been carefully studied in a few 

 cases, e.g. in guinea-pigs by Minot, and the facts are 

 striking. In guinea-pigs there is in both sexes a decline 

 in the growth-rate almost from the moment of birth. The 

 decline of rate is rapid from about the fifth day to about 

 the fiftieth ; from the fiftieth day onwards the decline 

 is slower, until the growth stops altogether. Of course the 

 animal is growing a great deal, and very quickly too, in its 

 early days, but the rate of growth gets less and less. More- 

 over, the post-natal decline in the growth-rate appears to 

 be a continuation of an ante-natal decline. As Dr. Jenkin- 

 son puts it, ' The younger the animal, the faster it grows ; 

 the more developed it is, the more slowly it grows. The 



