192 J. LIONELATANVTER [SEPTEMBER 
parts which at the time of fertilisation are somehow or other quali- 
tatively represented in the fertilised ovum. 
In development every organism passes through a series of stages 
which are more or less proportional to its specialisation and com- 
plexity, and the definite stages are passed through in a definite 
order, the highest specialisations, except where definite atavisitic or 
degenerative phenomena intervene, always coming at the later periods 
of development. When decay sets in in the organism we not uncom- 
monly find that this order is reversed, the higher being the first to 
disappear, just as they were the last to come. In the action of many 
drugs we see the same tendency ; if their action is general, the highest 
nerve-centres go first, the lowest fail last. Now this sequence in 
development, since it 1s so universal, must serve some purpose. The 
very early stages of segmentation appear to be little else than quanti- 
tative in character, but later qualitative differentiation begins to be 
manifested. The study of life in recent years has shown conclusively 
what an enormously important part the various products of tissue 
metabolism exert over life; the toxic and anti-toxic theories in disease, 
phenomena associated with internal secretion, the influence of vege- 
table alkaloids on different animal tissues, etc., all go to show that 
tissue activity is very dependent on its surroundings for its activity. 
Some facts of embryology lead to the conclusion that some organs 
have an almost purely developmental significance, and are of little use 
to the developed organism. We know also that organs vary in their 
relative importance and size to the whole organism at different periods 
of its development. How are we to explain the cause of this atrophy 
of some organs while others are developing, except on the assumption 
of a chemical food sequence? If we assume that, with a growing 
specialisation, itself induced by the liberation of metabolic products in 
the preceding stages, there is a growing specialisation of ferments and 
other material necessary to a more developed organism, and as a con- 
sequence a growing specialisation of all food material, we shall have a 
theory in accordance with facts, and which can explain many other- 
wise incomprehensible phenomena. The more specialised the food 
products circulating in the organism, the less favourable the conditions 
for the more generalised tissues; hence the progressive development of 
some tissues, and atrophy of others, would be explainable. 
The sequence in development would then be itself explainable, as 
the higher could only be developed from the lower through this 
sequence ; hence the necessity of recapitulation of the ancestral types in 
development. Rudiments would on this theory disappear in proportion 
to the generalised character of the rudiment as compared with organismal 
specialisation, and this would apply to germinal and somatic develop- 
ment. On this theory the whole organism would continue specialising 
so long as the morphological elements allowed of further differentia- 
tion; when this limit of specialisation was reached the organism 
