28 NATURAL SCIENCE. January. 



present. The process of development of all animals from the egg is 

 in all probability fundamentally the same kind of process in all cases, 

 and made up of the same factors, though some may be more 

 prominent in some cases than in others. 



Of course, everyone admits that there are numerous features in 

 ontogeny which are not due to ancestral repetition, but to subsequent 

 modification of the larva, and the question is how are the results, 

 due to these two factors, to be distinguished from one another. Now, 

 an important step towards attaining this result has been made by 

 Mr. Sedgwick in his theory of the relation to each other of the 

 embryonic and larval types of development. This theory is briefly as 

 follows : An embryonic stage of development is nothing but a larval 

 stage, which has been sheltered from the external world, by either 

 being enclosed in an egg-shell or retained in the body of the mother, 

 and modified in consequence. The larva retains ancestral features, 

 because it is subjected to ancestral conditions of life ; when, how- 

 ever, as in the case of reptile ontogeny, it has become converted 

 into the embryonic type, then a change, such as the loss of limbs, 

 when it occurs, can affect the development as a whole, and not even 

 embryonic vestiges remain, as is the case with snakes. On the other 

 hand, so long as the larva of the frog lives in water, it must retain 

 many fish-like features. I think, however, we should make a 

 mistake if we limited the influences tending to retain ancestral struc- 

 ture to the outer conditions, when these latter happen to be of 

 an ancestral type. It is to me impossible to suppose that the general 

 form ol insect larvae, the vermiform shape, and the large number of 

 almost similar segments, is not an ancestral feature ; but in view of 

 the extraordinarily diversified habits of these larvae, we cannot suppose 

 that the influence of outer conditions has retained it. If, however, 

 we say that the larval form is the combined result of outer and inner 

 conditions, we shall, I think, be nearer the truth. By inner con- 

 ditions I mean the intensity of the metabolism of an animal, corre- 

 lated with which is the differentiation from one another of the organs 

 fulfilling the various functions ; such a level of metabolism as the 

 adult possesses being only gradually obtained, when the said level is 

 a comparatively high one. Thus, when the long and comparatively 

 undifferentiated nervous system of the caterpillar becomes converted 

 into the concentrated nervous ganglia of the butterfly, there is no 

 doubt that we have reached a higher level of life. 



Suppose, then, that the ancestral features of ontogeny are due to 

 the repetition of ancestral outer and inner conditions, we can make a 

 rough estimate of characters which will not tend to be preserved by 

 these causes, but which, for the most part, will be obliterated by sub- 

 sequent modification. Chief among these is size : most larvae are 

 comparatively minute ; only in a few cases does the larval stage rival 

 in size the adult, while in very few cases (urodele amphibians) it tends 

 even to supplant it. Now, along with reduction in size goes, in all 



