at 
ee ee 
K.—BOTANY. 249 
‘as of use to the species that exhibited them, always with the implication, 
and sometimes with the express assertion, that they had been developed 
because of their survival values. One by one, in the light of critical 
research, most of these ‘ explanations’ of structure have broken down. 
Not only is ‘ survival value’ almost impossible to prove in any given 
case, but many of these supposed adaptive structures or arrangements 
have been shown not to work in the way they were supposed to work. 
Nevertheless, the habit has remained, even up to this day, not only of 
looking for the ‘use’ or ‘function’ of every structural character— 
quite a legitimate proceeding in itself, if we are not wedded to the belief 
that it must have a‘ use —but of considering its existence sufficiently 
‘explained’ when such a use has been experimentally established or 
even more or less plausibly suggested. 
Round about the beginning of the present century several publica- 
tions of first-rate importance began to put a new complexion on these 
problems. First there was De Vries’s work on mutations, '* which claimed 
to show that discontinuous variation, whose widespread occurrence in 
nature had already been demonstrated by Bateson and suggested by him 
as the prime cause of the discontinuity of species,’ was the important 
factor inevolution. In 1903 Johannsen’s work on ‘ pure lines ’* showed 
in the most unmistakable manner in the case of the bean that the mini- 
mal ‘ fluctuating ’ variations, on which Wallace and the Neo-Darwinians 
had been accustomed to rely as the material on which natural selection 
operates, are not inherited, so that if one breeds from a group of such 
variations which deviate from the mean of the pure line, there is no 
establishment of a deviating mean in the descendants, but a regression to 
the original mean. Meanwhile, the rediscovery of the Mendelian pheno- 
mena and the rapid extension of the range of characters in which they were 
found to be exhibited had at last placed our knowledge of the mechanism 
of heredity and variation on a secure basis. The immense quantity of 
breeding and cytological work which has followed has given reality to 
the conception of ‘ genetic constitution,’ or genotype as it is called in 
current terminology. We now know that an ordinary ‘ Linnean ’ 
species is, often at least, an aggregate or mixture of crosses from ‘ pure 
lines ’ in respect of different characters, each pure line with a specific 
genetic constitution based on the structure of the chromosome complex. 
New heritable variations of the stock are produced by redistribution of 
units within the chromosomes resulting from the crossing of individuals 
belonging to different pure lines or of their hybrid offspring. This 
apparently occurs in the stage of ‘ synapsis ’ of the nuclei which are just 
entering upon the divisions that result in the tetrads of spores and 
gametes ; and it is followed by the * reduction division ’ of the mother 
“cell of the tetrad, resulting in segregation of unlike units so that the 
gametes of a single tetrad bear different characters. Other internal 
changes in the chromosome complex may perhaps take place, but of 
these we can as yet say very little. 
10 Published in a serics of papers culminating in his great work, Die 
Mutationstheorie. Weipzig, 1901. Vol. II, 1903. 
11 Bateson, Materials for the Study of Variations. London, 1894. 
12 Johannsen, Ueber Mrblichkeit in Populationen und reinen Linien. Jena, 
1903. 
