with or without previous species-hybridization; by means of the sub-
traction or addition of whole chromosomes; or, in some cases, by
the breakage of chromosomes and the reunion of the pieces in new
The result is an overwhelming multiplicity of distinct species.
Naturally they are all adapted to their surroundings: but the geo-
graphical and cytological accidents that produced physical and genetic
isolation cause their number to be much greater than that which
would be necessary on purely adaptive grounds; and non-adaptive
variation adds its quota to the diversity.
Most of evolution is thus what we may call short-term, diversifica-
tion. But this kaleidoscopic change is shot through with a certain
proportion of long-term diversification in the shape of the long-range
trends revealed in fossils by the palaeontologist and deduced from
comparative studies by the morphologist. These trends are almost
all of them one-sided specializations, each one exploiting a particular
mode of life. Thus, both reptiles and mammals, beginning with small
and generalized creatures, radiated out into specialized lines includ-
ing carnivores, herbivores, climbing forms, flying forms, and aquatic
forms. Every possible niche is filled; some trends even involve de-
generation, such as the trend of the barnacles from a free-living,
shrimp-like creature to a sedentary life, or of other active Crustacea to
an existence as shapeless parasites.
These trends may continue for a very long time—up to tens of
millions of years: but they always come at last to a dead end. After
this, minor diversification may continue at the species level, but no
further improvement takes place in the major specialization. Thus,
birds ceased to show any improvement as flying mechanisms some
15 million years ago, and there has been no evolutionary improve-
ment of the ant type for perhaps 25 or 30 million years.
Such trends in a given direction are to be expected on Darwinian
principles. Improvement of teeth and claws for a carnivorous exist-
ence, for instance, will be an advantage to a small generalized
mammal when there are no specialized carnivorous mammalian com-
petitors already in the field, and will be favoured by natural selection.
And once the type has become at all adapted to flesh-eating, it will be
almost impossible for it to switch over to a herbivorous existence, for
example: the number of mutations needed is much too great, and
meanwhile any single mutation making for greater efficiency as a
carnivore will be caught in the net of natural selection and incorpor-
ated in the constitution of the stock. The stock thus finds itself at the
bottom of an evolutionary groove of specialization. Natural selection