THE PRINCIPLES OF BIOLOGY. 
181 
have here an answer to the question; an explanation of those 
causes which bring about the changed conditions. 
A most important matter is such differentiation of tissues 
as shall form organs for the production of seed, in order to 
perpetuate the species. The consideration of many facts in 
the study of vegetable physiology leads to the conclusion that 
the process of “gamogenesis commences where the forces 
that conduce to growth are nearly equilibrated by the forces 
that resist growth ; and the induction that in plants, fertilised 
germs are produced at places where there is an approach 
towards this balance is found “to be in harmony with the 
deduction that an advantage to the species must be gained by 
sending off migrating progeny from points where nutrition is 
failing.” 
The agency of insects in the propagation of seed has 
been pointed out by Darwin, and its importance fully illus¬ 
trated. Since Mr. Spencer wrote “ The Principles of Biology” 
much has been done in this fascinating branch of study, by 
many of our most eminent naturalists. Fertilisation of 
flowers, by means of bees and butterflies, is the most popular 
form of looking at the subject, but there are many insects 
other than these whose importance in this respect is quite as 
great. Whatever results have been obtained from these 
studies have, more or less directly, added proof to the things 
Mr. Spencer has said. 
Having dealt with the differentiations among the outer 
tissues of plants, we come to the consideration of “ The 
differentiations among the inner tissues of plants.” As we 
pass from plants of low organisation to those of a more 
complex structure, it is noticeable that the inner tissues begin 
to differentiate. The parts which were at one time homo¬ 
geneous now begin to assume a heterogeneity of structure, 
and this change may be noted increasingly as we ascend from 
low to high forms. In the lower forms, where, as an instance, 
the organism may grow in a vertical position, but to no great 
height, the differentiations in the inner tissues do not require 
to be very extensive. A spiral vessel will answer the purpose 
very well as in a fern, or other similar vessels as in the 
flower-stem of the blue-bell or tulip. When, however, a higher 
stage is reached, as the stem of an endogenous or exogenous 
tree, in order to keep it vertical, and in order to keep its 
branches in the right positions, considerable differentiation 
must have taken place. 
The function of circulation, and the formation of vessels, 
are matters which are part of this consideration. If at a 
certain place in a plant a large amount of sap is being con- 
