86 SCIENCE PROGRESS 



But it is evident that this is going further than there is any 

 need or right to go in biology, and I mention it only to 

 show to what a depth this law of unlikeness goes in natural 

 phenomena. Assuming such differences to exist, then however 

 they may possibly affect life, it is surely not in them that we 

 are to seek even the ultimate factors of the far greater differences 

 between one living being and another. P^or biological purposes 

 we must assume what is probably not true, that all atoms of 

 the same element are exactly alike. More, that each of the 

 highly complex chemical compounds, in which atoms are united 

 to form the bases of living matter, also consists of molecules, 

 of which every one is exactly like every other in weight, size, 

 shape and the arrangement of its component atoms. When 

 however we consider the manner in which the materials of 

 which a cell is built, or the units, be they what they may, 

 which compose those materials, are constructed out of these 

 theoretically uniform bricks, the chemical molecules, we must 

 suppose that at some stage or other structure comes into 

 play, and that the generally similar homologous parts of 

 two like individuals must differ ultimately in the number and 

 arrangement of the bricks of which they are composed. They 

 may no doubt differ in some degree in the kind of bricks, for 

 while all life is largely composed of the same chemical com- 

 pounds, and the homologous parts of like animals must be 

 almost entirely so composed, yet it is certain that some forms 

 of life contain chemical compounds absent from others, and it 

 seems not doubtful that two closely allied individuals, even 

 two animals of the same litter, apart from morbid changes, may 

 congenitally contain different chemical compounds resulting 

 in differences in pigmentation and other characters. But as 

 between animals of a kind this difference may be neglected ; 

 it is in the others that variations appear in the last analysis 

 to consist. Two like animals differ because their parts differ, 

 these because their constituent tissues differ, these again 

 because their component cells are unlike, and finally these 

 differ because either they, or the ultimate units of which they 

 are composed, are built of unlike numbers of like molecules 

 dissimilarly arranged. 



All this is of course far from being new, and it is mainly 

 speculative, having little inductive support derived from observa- 

 tion. What validity it may have is derived rather from necessi- 



