60 SECTIONAL ADDRESSES. 
continued to reproduce their own kind. Despite differentiation, the making 
of genera and species, of offshoots that lived and offshoots that failed, 
the Protozoa during more than 500,000,000 years have never given rise 
to anything but unicellular offspring. The conclusion is obvious, that in 
the Precambrian occurred a marvellous event due to certain conditions 
which have never since been duplicated. 
Similarly the primitive ccelenterate, presumably a sponge, gave rise 
to ancestral Cnidaria still within the Precambrian. Never since has the 
sponge given parentage to anything but the sponge, but the phylum has 
continued to exist and to differentiate within seemingly fixed bounds. 
Before the close of the Precambrian all the phyla of Invertebrata arose 
successively in this manner. Possibly we may include the vertebrates, 
although they have not yet been found so far back in time. These are well- 
known principles, trivial perhaps to an audience such as this, but re- 
iterated here because J think that they are not always given their true 
value. 
I would emphasise—the origin of phyla as great events in geological 
history, the crowding of these events into the Precambrian, the continua- 
tion of ancestral stocks, and their failure ever again to give rise to new 
phyla. It would appear, further, that higher phyla have not developed 
through highly specialised genera of lower phyla. For the invertebrates 
this is evident in the appearance of all the phyla in the Precambrian ; for 
the vertebrates, in the first place nothing is known with certainty, and in 
the second place the various phyla appeared long before high specialisation 
was attained by the ancestral stock. Amphibia arose from primitive 
Devonian ganoids, not from highly specialised teleosts; reptiles were 
derived from early Permian stegocephalians, not from highly specialised 
Anura or Urodela. The eutherian mammals appeared with startling 
suddenness in the Basal Eocene, and before the close of the period had 
developed into all the great classes. 
Evolution, in the phyletic sense, is not a gradual process, not uniform- 
itarian, but marked by great events in time. Specialisation and consequent 
fixation of characters are adverse to phyletic differentiation. 
It is apparent that phyla can arise only through genera and species. 
How far the phyletic principles may apply in the lower taxonomic ranks 
is an interesting question, the consideration of which would unduly extend 
this address. I would venture to state, however, that close adaptation 
(high specialisation) is likewise inimical to the production of new genera 
and species. Further, I believe that close adaptation is the main cause of 
extinction. 
Let us assume the existence of an organism perfectly adapted to its 
environment. Is it not a safe conclusion that any change in environment 
must result in the death of such organism ? That there is now or that 
there ever has been a perfectly adapted animal is extremely doubtful, 
but all animals must be more or less adapted or they could not exist. It 
may be stated that the margin between perfect and necessary adaptation is 
the zone'in which organic evolution is possible ; further, that the nearer 
an animal approaches perfect adaptation, the more liable it is to extinction 
on the advent of changed conditions. This conclusion is in accord with the 
generally recognised fact that in many instances highly specialised animals 
have suffered sudden extinction; it is also in accord with the general 
