1899] THE SCOPE OF NATURAL 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 groivth 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 

 growth, 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 applies 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, 



