368 SECTIONAL TRANSACTIONS.— D. 



relation is found to hold between body-weight and antler-weight as between body- 

 weight and chela-weight in fiddler-crabs. From this and from the facts known with 

 regard to the growth of imported Scotch deer in New Zealand, it can be confidently 

 asserted that the differences in antler-size between the deer of various geographical 

 areas are primarily due to environment, and not to genetic factors such as racial or 

 sub-specific differences. 



The same assertion can be made with regard to the so-called ' races ' of certain 

 insects. In Cyclommetus, a Lucanid beetle, five races or forms, each with a separate 

 name, have been distinguished according to the type of the mandibles. Analysis 

 shows that these are all parts of a continuous series ; the different ' forms ' are merely 

 determined by the absolute size attained at perfection, and have no genetic or taxo- 

 nomic significance. 



With regard to embryology, it should be pointed out that many examples of 

 recapitulation are direct consequences of differential growth of parts. In certain 

 Brachiopods the half-grown and young stages represent the adult stages of an earlier 

 and a much earlier form respectively ; this can be shown to be due solely to the new 

 type being produced by differential growth in certain regions of the shell. 



Heterogenic growth may equally well be negative in sign, i.e. the organ may 

 increase less rapidly than the rest of the body. This appears to be frequently the 

 case wiih vestigial organs ; in that case, wherever a vestigial organ recapitulates 

 past history by starting of greater relative size than that which it finally reaches, 

 the ' recapitulation ' may equally well be put down to negative differential growth 

 being the simplest biological method for producing an organ of small size. That 

 changes due to differential growth need have no evolutionary significance is seen in 

 the developments of the urinogenital systems of the two sexes in e.g. mammals from 

 a common type, by means of positive heterogeny of some parts, negative heterogeny 

 of others. If these changes were recapitulatory we ought to assert that the ancestral 

 mammal was hermaphrodite, which is highly unlikely. Thus recapitulation of growth- 

 stages may be only recapitulatory as an accident, the essential fact being the con- 

 venience of differential growth as a developmental mechanism. 



The most interesting facts from the evolutionary point of view are afforded by 

 the comparison of heterogenic organs in related species. Here, as G. W. Smith and 

 Lameere have shown, the same general law still holds, the species of absohitcly larger 

 bulk having the organ of relatively greater size. This has been shown to hold for 

 fiddler-crabs, for red deer, i.e. Wapiti, and for various insects. 



This has an important bearing upon theories of orthogenesis. In e.g. the classical 

 case of the Titanotheres, where, according to H. F. Osborn, several separate lines 

 independently develop horns and show subsequent increase of horn size, we need no 

 longer postulate independent change in the various lines of genetic factors controlling 

 horn-growth. All we have to postulate is (1) the existence within the group of a 

 developmental mechanism for heterogonic horn-growth similar to that shown to exist 

 for deer antlers. (2) an increase of absolute size during geological time ; this is probably 

 a biological advantage, and not itself an orthogenetic change. 



The other classical example of the Irish ' Elk ' (Cervus megac.eros) can be explained 

 on similar lines : increase of absolute size automatically caused relative increase of 

 antler-size to the limits of biological possibility. 



The study of relative growth of parts thus may throw important light on mor- 

 phology (absence of constant specific form or proportions) ; on taxonomy (invalidity 

 of certain so-called forms or races, which are in reality only size-variations, due 

 primarily to environment) ; on polymorphism (in ants, &c.) ; on embryology 

 (recapitulation) and on evolution (with regard to orthogenesis). 



23. Joint Discussion with Sections C (q.v.) and K on The Concept 



of a Species. 



24. Dr. K. E. Carpenter. — On the Toxicity of Lead-Salts to Fishes. 



Following upon the study of lead-pollution of rivers, a detailed investigation of 

 the action of lead-salts on fishes has been undertaken in the laboratory. The pre- 

 limi ary study had shown that amazingly dilute solutions of lead-salts might have 

 a fatal effect, and an elaborately standardised series of laboratory experiments has 

 rendered it possible to demonstrate mathematically that, accepting Powers' laws of 



