1899] THE SCOPE OF NATORAL SELECTION 127 
individual organisms competing in nature. Now this close relation of 
one part to another which is characteristic of the adult organism is 
also equally characteristic of the developing one, and, keeping this 
sequence of nutrition in view, each organism starting from a more or 
less quantitatively generalised substance, evolves to quantitatively 
specialised structure, in the building up of which every antecedent 
stage of development is necessary, and forms a basis for the later stages, 
it will follow that a definite, regular order will be developed; and 
hence definiteness in growth and development is as essential as definite- 
ness in the relation of one part of a specialised organism to another. 
That this necessary sequence in development is no mere unsupported 
conjecture is shown by the fact that the relation of parts alters with 
erowth, an organ occupying a first place in activity at one period may 
become second or third at another, this alteration of the relative size 
of different organs to the whole body at different ages must be of some 
value to the whole organism or it is unlikely that it would be perpetu- 
ated; the thymus gland affords a typical example of this—it appears in 
some way to be associated with development, it reaches its maximum 
size in man about two years after birth, and then slowly shrivels up ; 
the presumption is that at that period it had some function to perform 
which ceases to be required. If we assume a metabolic sequence in 
structure we explain this varying relation of parts, and we explain its 
definite character, and this sequence, as in other specialisations, would 
be subject to the influence of natural selection; so far preservation 
of different stages of growth can be easily accounted for on a selec- 
tion hypothesis if this necessary chemical sequence is assumed, and 
without it no theory has as yet explained the facts. 
There thus remains from this objection only those cases where 
there is an apparent or real foreshadowing of a higher evolutionary 
type. Now before this foreshadowing can be used as an objection, it 
has first to be determined how far it is real or not. It is well known 
that the ovum of one animal resembles another considerably, and that 
the higher animals, as they pass through successive stages of their 
development, resemble more or less incompletely certain lower forms of 
adult organisms, and this has led to the assumption of the recapitula- 
tion theory. Were it possible to reverse the order of evolution and 
proceed backward, we should find all types converging towards unity, 
and while this apples to the whole line of development, it equally 
applies to lesser portions of it. As the infant ape is less specialised 
than the adult ape it is more likely to present similarities to man, not 
on account of an actual foreshadowing, but simply because, being more 
generalised in structure, it is less easy to mark off differences ; for pre- 
cisely the same reason a human child might appear nearer to some 
ideal and higher type of man. 
Until this fictitious resemblance is dealt with this objection can 
be disregarded. Further, as many biologists have already pointed out, 
