THE EVIDENCE FROM DEVELOPMENT. 



235 



due to the fashion in which the original ciliated bands of the larva and 

 the embryo itself have been modified by the external and internal 

 forces which now, as of old, operate on living things. Professor 

 Lankester has suggestively worked out this idea of the derivation 

 of all existing embryos from a type-form, to which he has given 

 the name of " Architroch " a form represented by deputy, so to 

 speak, in certain worms and in the sea-mat class, as adult organisms. 

 Such a theory explains to us, on a basis of a reasonable nature, how 

 different forms may arise from a similar root-larva. And it may be 

 added, that should any objection be urged to such views on the 



ground that they _. 



are entirely hypo- 

 thetical, one may 

 retort that to all 

 other explanations 

 of the past of 

 nature, whether 

 theological or sci- 

 entific, exactly simi- 

 lar opposition may 

 be offered. Further, 

 we must reflect, that 

 in any case we have 

 to choose between 

 filling up from our 

 observation of na- 

 ture the gaps in our 

 knowledge which a 

 philosophical ne- 

 cessity entails, or 

 allowing these gaps 

 to yawn unsatisfac- 

 torily and perma- 



nentlyunfilled. The FlG " ^-DEVELOPMENT OF FROG. 



rational mind is not likely to hesitate in its choice of alternatives. 

 And if, lastly, it be borne in mind that, so far from being merely 

 shadowy theories, such ideas of the origin of animal forms are based 

 on close observation of nature often the work of many concentrated 

 lifetimes the logical standing of a theory which connects the facts 

 of nature, and by so connecting explains them, needs no justification, 

 as it fears no honest and unbiassed criticism. 



Turning now to the Vertebrate animals, we may find in the class 

 of frogs and newts (Amphibia) material for illustrating some of the 

 most important phases in normal development, and in -altered life- 

 histories as well. The life-history of a frog has already been alluded 



