THEORY OF DESCENT. 19 
not directly demonstrated by facts, namely, that the amount 
of variation may increase to any given extent in a sufficiently 
long time. But since the theory which involves this 
hypothesis is sufficient to explain the facts of morphology 
and adaptation, and since these are explained by no other 
scientific theory, we are justified in making the assumption.” 
If the differences between species are not due to descent 
‘with modification, then life must have originated many times 
upon the earth. Not a single reason can be adduced in 
favour of this, and all investigation into the ultimate 
structure and embryology of animals tends to prove the 
unity of all organised beings. 
But although we may feel sure that the differences 
between animals are due to descent with modification, we 
cannot at present find any adequate cause to account for the 
modifications. There is no known cause for the differentia- 
tion and specialization of function in cells; nor for the 
tendency which each ovum possesses to develop into a 
determinate form, nor is there any known cause for the 
origin of varieties. But, varieties being given, a cause is 
known which might in many cases make their offspring 
diverge until they became so different as to be regarded as 
belonging to different species or genera. This is called 
Natural Selection. Every year many more animals are 
born than can possibly live. This is obvious when we 
reflect that if all the young of any pair of animals lived 
their progeny would soon fill the world. The reasons that 
so many die are (1) inability to feed themselves, (2) inability 
to protect themselves. This gives rise to a struggle for 
life, the weak, the foolish, or the imperfectly adapted to the 
surrounding circumstances being the ones to suffer. Any 
animal possessing, by variation, better means of procuring 
food, or better means of protecting itself than are possessed. 
