446 DISCOVERY REPORTS 



A point of some interest arises when the development and growth of the whalebone 

 whales is examined in connection with the enormous size attained by the aduh. It has 

 already been pointed out that during the early part of gestation growth is slow, but that 

 the general form of the body is rapidly perfected, so that a foetus of 0-5 m. really differs 

 very little from an adult whale. 



Now although the difference in size between the sexes probably does not appear 

 until about the time when the young whale is weaned, reference to Figs. 154 and 155 

 will show that the difference in size between Blue and Fin whales is apparent quite 

 early in the development of the foetus. This specific difference in size is attained simply 

 by more rapid growth on the part of the larger species and not by growth spread over 

 a longer period. Blue whales are apparently ready for birth at a greater length in, if 

 anything, an actually shorter time than Fin whales. It is probable that in the early 

 stages of the development of the foetus, when the organs are being formed and the 

 limbs completed, the actual increase in length would be approximately the same for 

 both species, and it may be suggested that development up to this point does not differ 

 in any special way from the development of other mammals, and that the foundations 

 for the whale's great subsequent size have not yet been laid down. After this, however, 

 instead of development being quietly finished off and birth taking place, the rest of 

 gestation is devoted to a great burst of growth, the rapidity of which in the different 

 species appears to be proportional to the size of the whale when fully adult. As it is 

 practically certain that the great size of whales is, from the evolutionary point of view, 

 a recently acquired character, it would naturally be expected to make its appearance in 

 the later part of gestation. Thus the great size of a whale does not necessarily imply the 

 need for a long period to attain that size. The capacity for rapid growth is to be re- 

 garded rather as one of a number of characters distinguishing certain whales from other 

 mammals. 



THE AGES OF WHALES 



It is important that something should be known of the ages of whales, but the problem 

 is a very difficult one to approach. At present no direct method of judging the age of 

 any individual has been found, but it is often possible to say whether one whale is older 

 or younger than another, and in the case of the younger whales there are sometimes 

 grounds for making some kind of guess at the actual age. The main object, however, 

 of this section will be to give an idea of the kind of results which may be hoped for in 

 this direction in the future. 



The size of a whale, the number of old scars, the condition of the vertebral epiphyses 

 and the number of old corpora lutea may all throw some light on the age of a whale. 



It is obvious that size is up to a point a rough criterion of age, and we already have 

 grounds for supposing that when a whale reaches the size at which it should become 

 adult it is about two years old. After a whale becomes adult it may reasonably be sup- 

 posed that it will continue to grow at least a little and that in some cases it adds several 

 metres to its length, so that one is justified in saying that, for instance, any female 

 Fin whale measuring about 20 m. is unlikely to be more than two or three years 



