392 SECTIONAL TRANSACTIONS.—D. 
long history behind them. The lines which are most productive in new 
species seem to be comparative newcomers which burst into activity under 
conditions which may have been exceptionally favourable. It is also note- 
worthy that the species appear to be most abundant when they, as judged 
by the above prolific characters, seem to be at the height of their vigour. 
Dr. C. C. Hurst. 
Genetic experiments lead to the conclusion that four prime factors are 
concerned in the origin of species: (1) Mutations of genes. (2) Trans- 
mutations of chromosomes. (3) Sex. (4) Natural Selection. 
It is now definitely established that the living molecules known as genes, 
which are linked together in the chromosomes in the nucleus of every growing 
cell, are the organisers and determiners of all structural and functional 
characters and the transmitters of these from one generation to another. 
Random changes in the genes and chromosomes, brought about by high- 
frequency radiations and irregular divisions, are the source of almost all 
heritable variations and the gene and chromosome complex constitutes the 
mechanism of heredity, variation, individuality and evolution. 
The importance of sex cannot be overestimated as a means of segregation, 
recombination and distribution of new characters. So long as these are 
not lethal to the species they will persist under natural selection, giving a 
great range of variation. 
Natural selection is not a primary cause of evolution, but it is the final 
arbiter and determiner of the survival of species in nature. Man, by the 
exercise of his intellect, can replace natural selection by human selection, 
not only with plant and animal species, but also with his own species. 
Dr. W. M. WHEELER and Dr. N. A. WeBer.—Unusual prevalence of 
sex-mosaics (Gynandromorphs) in a colony of a Trinidad Ant (12.0). 
During the spring of 1935 Dr. Weber found nesting in the trunk of a 
large saman tree in Trinidad, B.W.lI., a large colony of Cephalotes atratus L. 
containing more than 2,600 sex-mosaics, or 27:9 per cent. of the population. 
All these anomalies exhibit combinations of worker (sterile female) and 
male characters. In most of the individuals the sexual mosaic pattern 
is confined to the head, and the thorax, legs and abdomen are of the 
normal worker type, but several also exhibit admixture of male abdominal 
characters. The occurrence of so many sex-mosaics in a single colony 
and as the offspring of a single mother is remarkable, because very few 
ants exhibit such striking structural differences as Cephalotes between the 
normal male and worker, and because since 1854 less than 75 gynandro- 
morphs have been observed among the hundreds of thousands of ants 
that have been studied by myrmecologists. Nor is the proportion of sex- 
mosaics to normal individuals greater among other insects in nature. Under 
artificial conditions the proportion, especially in hybrid cultures, is greater, 
but still very far from comprising a fourth of the population. 
The queen mother of the Cephalotes colony is perfectly normal, with 
large ovary containing many eggs in all stages of growth. In sections 
none of these eggs shows any indication of binuclearity or of having arisen 
by fusion of two oocytes. This case, therefore, fails to support the 
hypotheses of the origin of gynandromorphism advanced by Dénhoff, 
Wheeler, Doncaster and von Lengerken. The colony lived under con- 
ditions which seem to preclude the effects of hybridism, parasitism, low 
temperatures, X-rays, ultra-violet rays or mechanical shock, which, 
