62 SECTIONAL ADDRESSES. 
would render the new species liable to extinction on the recurrence of 
unfavourable conditions. 
This explanation of the common failure of vertical seriations emphasises 
migration as a factor in evolution and leads to the conclusion that tran- 
sitional stages are few in number, scattered over wide geographical 
extents, and disposed in stratigraphically oblique lines. Barrell’s 
‘ diastems,’ to be referred to later, support this explanation of abrupt 
changes in the faunas. 
The line of division between animals and plants is not clearly defined, 
but in a general way the distinction is clear enough for our purposes. The 
evolution of plants is shown chiefly in an increasing complexity of structure, 
but that of animals is dual—increasing complexity of structure and more 
and more acute sensitive discrimination and response. The evolution of 
the nervous system is only one expression of the increasing complexity of 
structure, but it runs more or less parallel to an increasing ‘ sensibility ’. 
I have no intention of entering the discussion as to the relation of mind 
and matter, but wish merely to point out that there has been an evolution 
of sensibility as well as an evolution of the physical organism, using the 
term ‘evolution’ without the precise definition that is demanded by 
psychologists. 
Various terms such as ‘ instinct,’ ‘ intelligence,’ ‘ mentality ’ have been 
applied to certain stages of mental development. As parts of an evolu- 
tionary series it is evident that these terms cannot be defined rigidly. 
Nevertheless, if a comparison with the physical development is justified, 
we should expect to find that the successively higher stages of mental 
capacity appeared with some degree of abruptness. 
To my mind the most striking difference between the physical and 
mental development lies in the cumulative nature of the latter, a feature 
which is in accord with Joseph’s contention that the term ‘ development ’ 
is justified only with regard to the mental series. In man, for instance, 
higher reasoning has not replaced intelligence, nor has intelligence replaced 
instinct, nor has instinct replaced mere nervous reaction and response. 
Doubtless certain nervous attributes have been lost in the higher animals, 
for vestigial sensibilities can be found as well as vestigial organs. 
The evolution of sensibility is not necessarily parallel to that of the 
physical structure of organisms. Insects are nervously endowed in excess 
of their structure, and the mentality of man is out of all proportion to his 
physical equipment. Can it be inferred that mental development is the 
indicated road for further progress? Does the evolutionary series of 
sensibility begin with a protoplasmic response to stimulus and end with 
omnipotence, and does man occupy a position an eternity from the starting 
post and another eternity from the goal ? 
In the preceding remarks I have expressed no opinion as to how or 
why evolution has been carried on; personally I incline strongly to the 
vitalistic creed and the Bergson philosophy. My purpose has been to 
emphasise certain conclusions from geological observation that appeal to 
me as fundamental and which may be summarised as follows : 
1. The law of tendency to the complex. This is a mere statement of 
observed fact. 
2. The tendency to the complex is not a force acting on all organised 
matter. It comes into play only under especial conditions, 
