ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
609 
the Selachian retina and that of Amphibians, but divergence from that 
of Teleosteans. Thus in Teleosteans, as in nocturnal birds and in 
mammals, the rods have a terminal knob in the external plexiform layer ; 
while in Selachians many of the rods have a small conical termination, 
giving off horizontal processes, as all do in Amphibians and diurnal 
birds. 
Again, the Selachian retina is distinguished from that of Teleosteans 
by the presence of certain bipolar cells (the internal bipolar cells of 
Eamon y Cajal). These occur in Ganoids and Amphibians, less abun- 
dantly in reptiles and birds, and even in man, but not in Teleosteans. 
It is probable that these cells may act as physiological complements of 
the rods and cones whoso numerical proportions are well known to be 
variable. 
7. General. 
Age of the Earth. * — Prof. E. B. Poulton, in his Presidential Address 
to Section D of the British Association, took as the subject of his dis- 
course the difficulty raised by Lord Salisbury in his Presidential Address 
at Oxford in 1894, as to the age of the earth. It will be remembered 
that Lord Kelvin has considered one hundred million years as the time 
during which the earth has been in a habitable state, while Prof. Tait 
concedes only ten million years. Prof. Poulton points out that Darwin, 
Huxley, and Herbert Spencer have all independently agreed that the 
time during which the geologists concluded that the fossiliferous rocks 
had been formed was utterly insufficient to account for organic evolution. 
Dealing first with the arguments of the physicists, it is shown that the 
objection raised by the evidence afforded by tidal retardations has in it a 
flaw so serious as to make it of no import. With regard to the evidence 
afforded by the cooling of the earth, physicists themselves appear to be 
in complete disagreement. The third argument, which depends on the 
life of the sun, would appear to have greater weight, but the astronomers 
allow five hundred million years as the maximum life of the sun. 
Turning next to the geological argument, it is pointed out that Geikie 
demands a period of nearly four hundred million years, and is inclined 
to ask for four hundred and fifty million. 
The biological evidence as to the age of the earth is next considered, 
and it is pointed out that the ancestor of any one of the nine higher 
phyla of animals must have passed through an immense evolutionary 
history. The changes that have occurred must have taken an immensely 
long time, for all available evidence points to the extreme slowness of 
progressive evolutionary changes in the Coelentera ; while the relatively 
ancestral line, at every stage of its complex history of originating some 
higher line, itself continued down to the present day throughout the 
whole series of fossiliferous rocks with but little change in its general 
characters, and practically nothing in the way of progressive evolution. 
Fundamental facts in structure and development may remain changeless, 
amid endless changes of a more general character. In trying to get some 
conception of the amount of evolution which has taken place in the higher 
phyla of the Animal Kingdom during the period the fossiliferous rocks 
were deposited, Prof. Poulton deals as briefly as may be with a number 
Nature, liv. (183G) pp. 500-9. 
