56i SECTIONAL TRANSACTIONS.— D. 



By expressing thickness of blubber as a percentage of the total length of the whale, 

 comparison has been made between the average thickness of all whales of the same 

 species and sex month by month, showing that for Blue and Fin males and non- 

 pregnant females there is an increase in thickness as the season advances at South 

 Georgia and a decrease at Saldanha Bay, South Africa. 



Pregnant females are always fatter than normal. They also appear to get fatter 

 at the end of the South Georgia season. Laetating females captured early in the 

 season (June) at South Africa are extremely fat ; but when they appear at South 

 Georgia in the second half of the season they are very much thinner than normal. 



The records throw some light on the migrations of mature and immature whales, 

 on the time of birth, and the duration of lactation. 



Prof. F. Balfour Browne.— Ow the Habits of certain Social Caterpillars. 



Social insects are those in which the members of the family do something for the 

 common good, and gregarious caterpillars which spin silk come within this definition. 



There are three main types of web spun by social caterpillars, the ' Carpet-web,* 

 spread over the food and eaten with the food, the ' Feeding-web,' spread over the food, 

 the caterpillars living inside it and expanding it as the food is eaten, and the ' Home- 

 web,' from which the caterpillars go out to feed. 



Two examples of British ' Home-web ' builders are the Lackey Moth caterpillar 

 (Clisiocampa neustria) and the Little Eggar Moth caterpillar (Eriogaster lanestris). 

 The former species is in an elementary stage, the primary web being quickly deserted, 

 the family breaking up into smaller batches, each batch forming a secondary web and 

 the batches later breaking up into soKtary individuals. 



The Eggar, on the other hand, is much more advanced, and experiments show that 

 the original home is retained even under adverse circumstances, and that, in the 

 absence of the original home, the family tends to remain together and adopt a 

 secondary centre. The general characters of the Eggar web, the method of con- 

 struction and its uses, are of some interest ; the web seems to be essential for the 

 well-being of the species. The migratory instincts of the full-grown caterpillars are 

 remarkable. 



Mr. J. T. Cunningham.— 06/ec<^■ons to the Mutation Theory of Evolution. 



Inherited characters must be determined by the constitution of the chromosomes 

 of the reproductive cells or gametes. Hereditary changes in these characters must be 

 due to changes in the constitution of the chromosomes. According to T. H. Morgan's 

 theory, the characters are determined by particles of the chromosomes arranged in 

 linear series. A new character or mutation, such as double wings in a culture of 

 Drosophila, is the effect of a spontaneous change in the corresponding gene. But in 

 order to explain evolution it is necessary to consider (a) the course of the development 

 of the final character, (b) the relation of the character to function, i.e. to the external 

 environment. According to Morgan's views, all such matters are adequately explained 

 by natural selection, and it is assumed that the external environment plays no part 

 in causing hereditary mutations in particular directions. 



It may be admitted that the conception of mutation harmonises well with our 

 knowledge of characters which develop directly and which have no known function 

 or utility, but it is impossible to accept the process of selection as sufficient explanation 

 of the relation between the structure and functions of the animal and the matter and 

 energy of its surroundings. This is especially evident in the phenomena of metamor- 

 phosis and recapitulation, and in those of sex-hmited characters. In these cases 

 the processes of development are influenced and controlled by internal secretions. 

 It is difficult to beheve that the transition during the life of the individual from 

 aquatic to atmospheric respiration in Vertebrates was due to the mere selection of 

 mutations unrelated to the respiratory medium, and still more difficult to understand 

 how the theory of mutation and selection can account for the influence of the thyroid 

 secretion on this metamorphosis. It is equally difficult to believe that the relations 

 of sex-limited characters such as the antlers of stags to the external function of 

 weapons of sexual combat, and to the internal influence of the hormones produced by 

 the sexual organs, have no other explanation than that of selection of mutations in 

 their origin undetermined by these external and internal relations. Morgan's sug- 

 gestion that the antlers of stags could be explained as a mere by-product of th» 



