ORGANIC DIFFERENTIATION 95 



not hitherto been possible to extract from embryogeny all 

 the data it affords on the subject of the origin of living forms ; 

 for this reason no light has been shed in particular on the 

 perfectly commonplace causes which determine the formation 

 of the great organic types. Indeed, so accustomed have we 

 become to regarding these facts as miraculous phenomena 

 that the explanations we shall make are liable to the reproach 

 of being over-simple. Mystical or purely verbal explanations 

 have been treated with more respect, as if there were any causes 

 operating around us other than commonplace causes. The world 

 tends to forget that all the progress we have made in geology 

 within the last few generations is due to the abandonment of 

 old doctrines based on miraculous cataclysms, universal floods, 

 and other " world revolutions ", in favour of the careful 

 investigation of actual cause and effect, to which Buff on and 

 Lamarck had already devoted themselves before Sir Charles 

 Lyell systematized them. 



The oldest known geological period, the Archaean, was 

 actually of longer duration than the whole Primary Period, and 

 witnessed the formation of deposits which are still more than 

 20,000 metres thick, despite the levellings and metamorphic 

 transformations they have undergone. Yet if we wish to recon- 

 struct the forms of life which probably prevailed so long ago, 

 the inadequacy or, indeed, the total absence of palaeontological 

 documents compels us to fall back upon the data supplied 

 by existing forms. As we have already observed, the lowest of 

 these are so simple that we can hardly conceive of any simpler. 

 If there be any general theory which establishes a connexion 

 between these simple beings and the most complicated 

 organisms we know, and leaves no gaps in the chain, that theory 

 will possess the best chance of being applicable to fossil as 

 well as to living forms. Consequently it will add precision 

 to our work in linking the first with the second, interpreting 

 the inadequate remains left to us, and filling in the lacunae 

 between the forms that have persisted. It will also frequently 

 guard us against judging by appearances in determining the 

 date when these forms first appeared. Such a theory would be 

 synonymous with an explanatory genealogy of living forms, 

 which we are now about to outline. This question can only be 

 broached, however, after the principles we have just expounded 

 have been well established. 



