THE LAW OF EVOLUTION CONTINUED. 381 



" epiphyses." Generally, indeed, we may say that increase 

 of definiteness continues when there has ceased to be any 

 appreciable increase of heterogeneity. And there is reason 

 to think that those modifications which take place after 

 maturity, bringing about old age and death, are modifica- 

 tions of this nature; since they cause rigidity of structure, 

 a consequent restriction of movement and of functional 

 pliability, a gradual narrowing of the limits within which 

 the vital processes go on, ending in an organic adjustment 

 too precise too narrow in its margin of possible variation to 

 permit the requisite adaptation to changes of external con- 

 ditions. 



133. To prove that the Earth's Flora and Fauna, 

 regarded either as wholes or in their separate species, have 

 progressed in definiteness, is no more possible than it was 

 to prove that they have progressed in heterogeneity: lack 

 of facts being an obstacle to the one conclusion as to the 

 other. If, however, we allow ourselves to reason from the 

 hypothesis, now daily rendered more probable, that every 

 species up to the most complex, has arisen out of the simplest 

 through the accumulation of modifications upon modifica- 

 tions, just as every individual arises ; we shall see that there 

 must have been a progress from the indeterminate to the 

 determinate, both in the particular forms and in the groups 

 of forms. 



We may set out with the significant fact that the lowest 

 organisms (which are analogous in structure to the germs 

 of all higher ones) have so little definiteness of character 

 that it is difficult, if not impossible, to decide whether they 

 are plants or animals. Respecting sundry of them there are 

 unsettled disputes between zoologists and botanists; and it 

 is proposed to group them into a separate kingdom, forming 

 a common basis to the animal and vegetal kingdoms. Note 

 next that among the Protozoa^ extreme indefiniteness of 

 shape is general. In sundry shell-less Rhizopods the form is 



