WHALEBONE WHALES—MACKINTOSH 255 
Ruud (1940, 1945) describes an important new method of determin- 
ing the age of a whale from delicate measurements of the thickness 
of the baleen plates. From gum to tip the thickness decreases in a 
series of levels, or steps, and there seems little doubt that these repre- 
sent years of age. The levels are sometimes hard to distinguish, and 
difficulties arise through the wearing away of the plate at the tip. 
The method should be of much value for determining the ages of 
young whales, and gives good reason to believe that sexual maturity 
is generally reached 3 years after birth in fin whales in the Northern 
Hemisphere, but further investigations will be needed to test its re- 
lability as a check on the rate of accumulation of corpora lutea in 
adult whales. 
A final point to be mentioned in connection with the growth and 
age of whales is the relative growth rate of different parts of the 
body. From measurements of the external proportions of a large 
number of blue and fin whales Mackintosh and Wheeler (1929) found 
that in both these species the rate of growth is faster in the anterior 
than in the posterior part of the body. For example, in female blue 
whales 13.5 m. in length the head on the average measured 15 percent 
of the total length, and the tail region 33 percent, but in those measur- 
ing 26.5 m. the corresponding figures were 21 and 28.5 percent. Mat- 
thews (1937) found a similar effect in the humpback and suggested 
that the relative increase in the size of the head is connected with 
the development of the feeding mechanism represented by the whale- 
bone and mouth. In the sei whale, however, he found a more even 
growth rate which he thought to be correlated with the smaller size of 
the species and the lower food requirement. According to Esricht 
and Reinhardt (1861) the head of the Greenland right whale is 
slightly less than a third of the total length at birth and slightly more 
than a third in the grown whale. D’Arcy Thompson (1919) found 
that in fin whales the longer whales had a proportionately larger girth. 
V. POPULATIONS AND THE EFFECT OF THE WHALING INDUSTRY 
Whales are more or less gregaricus animals, and although solitary 
members of any species are commonly seen they frequently swim in 
schools of two or more in which they keep within a few yards of one 
another. Sometimes a number of schools and individuals are found 
in a limited area, forming what may be loosely termed a herd of 
whales, but they are often widely dispersed, and it is perhaps best to 
apply the term “population” only to large groups.of whales, such as 
the fin whales found in Antarctic waters in summer, or one of the 
principal southern communities of humpbacks. 
Although no reliable estimate has yet been made of the actual num- 
bers of whales which make up the population in any region, informa- 
