TRANSACTIONS OF SECTION D. 4G9 



to say what is the direct influence of any one physical factor on tlic iiisei t 

 association, but it is hoped by making a study of an entirely different area 

 to reach important conclusions by the method of comparison. Probably it will 

 be found that the best index of the physical factors of an environment may 

 be derived from the ' evaporating power of air ' as measured by the porous 

 cup atmometer. 



The index of an insect's habitat is where it breeds, which may not be the 

 place where it feeds. Many insects migrate from one vertical stratum to 

 another at different periods of their life-histories or according as they ai-e 

 actuated by their various needs. Some insects are peculiar to an association, 

 others invade from adjacent associations. An analysis of their habits, which 

 involves a knowledge of their larval habits, enables one to relegate them to 

 their proper association. A tabular statement of these facts has been presented 

 in the paper, and the various orders of Apterygota, Plecoptera, Neuroptera, 

 Mecoptera, Trichoptera, Diptera, Coleoptera, Lepidoptera, Hymenoptera, and 

 Hemiptera as specifically determined have been reviewed. 



A soil insect-census of two different grasslands in the association, differing 

 in their soil-types and vegetational covering, .showed that in any given locality 

 the soil-insect fauna of grassland is not likely to vai-y to any great extent. Foi' 

 a determination of the faunistic differences on the basis of the variation of 

 edaphic factors, a very thorough analysis is necessary. 



10. Exhibition of Lantern Slides of Maps illustrating Tertiary Changes, 

 according to Geologists, in the Northern Hemisphere, and the 

 Light thrown on the Subject by a Comparison of Maps showing 

 Distribution of Fish. By Professor A. Meek. 



THURSDAY, SEPTEMBER 9. 

 The following Papers were read : — 



1. Discussion on the Relation of Chromosomes to Heredity. — Opened 

 by Professor E. W. MacBride, F.R.S. 



There seems to be no escape from the position that the chromatin viewed as 

 a whole is the bearer of the hereditary tendencies, for the influence of the 

 father in determining the character of the offspring is as potent as that of the 

 mother. Now the head of the spermatozoon is the only part of the father 

 wliich enters into the constitution of the progeny, and this appears to consist 

 practically exclusively of chromatin (Duesberg). This chromatin appears 

 after fertilisation in the somatic nuclei of the embryo as a group of chromo- 

 somes, which are in many cases distinctly separable into two sub-groups of 

 maternal and paternal chromosomes respectively. These can only influence the 

 characters of the embryo by emitting substances which pass through the nuclear 

 membrane into the cytoplasm. The formation of the organs of an embryo is 

 known in many cases to be due to substances localised in the cytoplasm. But 

 the formation of these substances can be shown to be due to chromatic emission 

 from the nucleus of the unripe egg (Schaxel), and in many cases where the 

 fertilised egg exhibits marked cytoplasmic differentiation the cytoplasm of the 

 unripe egg can be shown to be of homogeneous composition (Boveri). Driesch 

 has shown that in some cases the nuclei of an embryo can be displaced from 

 their positions without altering the mutual relations of the organs of an embryo. 

 From this it is argued that the nuclei are all alike and indifferent. But during 

 this period the embryo is of uniform character, i.e., all its parts are equi- 

 potential. When this period has passed, and definite localisation of the rudi- 

 ments of organs begins, this is associated with renewed emission of chromatin 

 into the cytoplasm (Schaxel). 



Whether individual chromosomes are the bearers of different characters or 



