SECTIONAL TRANSACTIONS.—D. ‘ 397 
the anterior pair in the Tornaria larva, but are carried back by interstitial growth 
exactly as in Amphioxus as the trunk lengthens, but they are not segmented into 
somites. 
The earliest traces of Vertebrates are found in freshwater beds; it is strongly 
indicated that the segmentation of the mesoderm, which is a diagnostic feature of 
the vertebrates, was brought about when the ancestral marine group entered the 
rivers and had to maintain themselves by vigorous lateral blows of the body against 
being swept away by the current. E 
Mr. J. H. Woopcer.—Germ Layer Formation in Birds. 
Prof. J. GRauAmM Kerr, F.R.S.; Mr. G. L. Purser, and others. 
Saturday, September 26. 
Discussion on Population :— 
Prof. J. 8. Huxtey. 
Prof. A. M. Carr-SaunpERS.—The Population Problem. 
Prot... T. Hoapen.—Experimental Biology of Population Growth. 
Prof. J. B.S. Hatpane.—Some Causes and Consequences of Differences 
in Viability. 
GENERAL Discussion. (Prof. E. W. MacBripg, F.R.S.; Prof. 
F. A. E. Crew.) 
Prof. E. W. MacBrivg, F.R.S8.—The Problem of Population. 
It is a commonplace that in Nature every species of animal produces so many 
offspring that in a few years, if all survived and reproduced as did their parents, every 
species would overrun the entire globe and crush out every other form of life. Thus 
a pair of thrushes in ten years, which is the normal life of a thrush, would produce 
19,000,000 offspring, whilst the brood produced annually by a pair of herring would, 
if they all survived, cause the herring population to increase 5,500-fold a year. 
Whilst in general natural selection limits the increase so that the constituents of the 
population remain relatively constant, occasional enormous increases in the number 
of certain species take place. The age of herring can be computed by counting the 
rings on the scales. When this is done it is found that the herring hatched in one 
particular year may supply the bulk of the catch landed for four years in succession. 
The same is true of other food-fish. In cases such as that of the plaice, where the 
eggs float, the average number of eggs laid each year can be reckoned; it is then 
seen that ‘a good fishing year’ is due not to an increased birth-rate, but to an 
increased survival rate, because in certain years an increased supply of microscopic 
plants is present to provide food for the young fry when the maternal food-supply is 
exhausted. The highest mortality takes place at this point. If a fish survives this 
crisis its expectation of life is enormously increased. 
In land animals the same periodic increases in population occur due to the same 
causes: the overcrowding resulting from them acts as a stimulus to mass migration, 
which produces devastating effects. Thus Uvarov has shown that an ordinary harmless 
grasshopper becomes changed into a devastating locust. 
The study of human history discloses the fact that periodic migrations of human 
tribes were common in ancient times, and that civilisations, notably the Roman 
civilisation, have been swept away by them. Tradition indicates that these migra- 
tions were due to periodic increases in population. One of the most notable increases 
in population which has occurred in comparatively recent times has taken place in 
our own island, and, as everyone knows, it has been accompanied by an enormous 
