PRESIDENT’S ADDRESS. 5 
first to provide a body of fact demonstrating the variability of living 
things, whatever be its causation, can never be questioned. 
There are some older collections of evidence, chiefly the work of 
the French school, especially of Godron?—and I would mention also 
the almost forgotten essay of Wollaston *—these however are only 
fragments in comparison. Darwin regarded variability as a property 
inherent in living things, and eventually we must consider whether this 
conception is well founded ; but postponing that inquiry for the present, 
we may declare that with him began a general recognition of variation 
as a phenomenon widely occurring in Nature. 
If a population consists of members which are not alike but differen- 
tiated, how will their characteristics be distributed among their off- 
spring? This is the problem which the modern student of heredity 
sets out to investigate. Formerly it was hoped that by’ the simple 
inspection of embryological processes the modes of heredity might be 
ascertained, the actual mechanism by which the offspring is formed 
from the body of the parent. In that endeavour a noble pile of 
evidence has been accumulated. All that can be made visible by 
existing methods has been seen, but we come little if at all nearer to 
the central mystery. We see nothing that we can analyse further— 
nothing that can be translated into terms less inscrutable than the 
physiological events themselves. Not only does embryology give no 
direct aid, but the failure of cytology is, so far as I can judge, equally 
complete. The chromosomes of nearly related creatures may be 
utterly different both in number, size, and form. Only one piece of 
evidence encourages the old hope that a connection might be traceable 
between the visible characteristics of the body and those of the chromo- 
somes. I refer of course to the accessory chromosome, which in many 
animals distinguishes the spermatozoon about to form a female in 
fertilisation. Even it however cannot be claimed as the cause of 
sexual differentiation, for it may be paired in forms closely allied to 
those in which it is unpaired or accessory. The distinction may be 
present or wanting, like any other secondary sexual character. Indeed, 
so long as no one can show consistent distinctions between the 
cytological characters of somatic tissues in the same individual we 
can scarcely expect to perceive such distinctions between the chromo- 
somes of the various types. 
For these methods of attack we now substitute another, less 
ambitious, perhaps, because less comprehensive, but not less direct. 
If we cannot see how a fowl by its egg and its sperm gives rise to 
? De VEspéce et des Races dans les Btres Organisés, 1859. 
* On the Variation of Species, 1856. 
