514 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
and possesses a lumen which arises as an extension of the coelom itself. 
Into the details of the various groups which he describes it is impossible 
for us to follow him, but we may note that he concludes in the following 
terms : — the coelom can be traced from its smallest beginning as a cavity 
or cavities in which are developed the gonad-cells ; it grows gradually 
in size and importance until it becomes the body-cavity in which the 
viscera rest ; the genital ducts, with a few possible exceptions due to 
secondary modifications, are homologous throughout the Coelomata ; the 
nephridia, which have often been confused with these ducts, can always, 
when they occur, be distinguished from them ; and finally, the coelom 
may secondarily acquire a renal function, in consequence of which 
the peritoneal funnels supersede the nephridia proper as excretory 
ducts. 
Variation in Animals and Plants.* — Prof. W. F. R. Weldon, as 
Secretary to a Committee of the Royal Society appointed to conduct a 
statistical inquiry into the measurable characteristics of animals and 
plants, has prepared the first report. In this it is pointed out that 
while the importance of variation as a factor in organic evolution is not 
seriously disputed, naturalists are not agreed as to the manner in which 
variation among individuals is associated with the modification of species. 
The original view of Darwin and Wallace was that specific modification 
is, at least generally, a gradual process, but of late years another view 
has received support from various writers. It has been assumed that 
the advantages or disadvantages which accompany the more frequent 
slight abnormalities are in themselves of necessity slight, and that the 
effect of such slight abnormalities may be neglected. These writers 
regard change in specific character as an event which occurs occasionally 
and by steps of considerable magnitude. Prof. Weldon, however, 
discusses the effect of small variations. It is further pointed out that 
the questions raised by the Darwinian hypothesis are purely statistical, 
and the statistical method is the only one at present obvious by which 
that hypothesis can be experimentally checked. In order to estimate 
the effect of small variations upon the chance of survival it is necessary 
to measure, first, the percentage of young animals which exhibit this 
variation, and secondly, the percentage of adults in which it is present. 
When the law of growth has been ascertained the rate of destruction 
may be measured, and in this way an estimate of the advantage or dis- 
advantage of a variation may be obtained. Numerical data of the kind 
indicated contain all the information necessary for a knowledge of the 
direction and rate of evolution, and it is strongly urged that the 
importance of such numerical statements in testing the current theories 
of adaptation, &c. is great. Mr. Weldon’s report on the variation in 
the shore-crab is noticed on another page.f 
Discontinuous Variation.! — M. H. de Vries distinguishes fluctuating, 
individual or continuous variation — modifying the relative number of 
different kinds of pangenes — from specific variation, in which the division 
of pangenes is qualitatively diverse. The fundamental idea of his theory 
of pangenesis is that the diverse hereditary qualities have materially 
* Proc. Roy. Soc., lvii. (1895) pp. 379-82. f See p. 527. 
t Arch. Neerland. Sci., xxviii. (1895) pp. 442-57 (1 pi.). 
