DEVELOPMENT OK ANIMAL LIFE IN TIME 3O3 



The whole of this argument was based upon M. Vogt's examina- 

 tion of the development of the Coregonus, one of the Salinonidce , the 

 tail of Coregonus being found to pass through a so-called heterocercal 

 state in its passage to its perfect form.^ For the argument to have 

 any validity, however, two conditions are necessary, i. That the 

 tails of the Sabnonidce should be homocercal, in the same sense as 

 those of other homocercal fish. 2. That they should be really hetero- 

 cercal, and not homocercal, in their earliest condition. On examina- 

 tion, however, it turns out that neither of these conditions holds good. 

 In the first place, the tails of the Salinonidce, and very probably of 

 all the Physostomi are not homocercal at all, but to all intents and 

 purposes intensely heterocercal : the chorda dorsalis in the Salmon, for 

 instance, stretching far into the upper lobe of the tail. The wide 

 difference of this structure from true homocercality is at once obvious, 

 if the tails of the Salmonidce be compared with those of Scomber 

 sconibrus, Gadus ceglefinus, &c. In the latter, the tail is truly homo- 

 cercal, the rays of the caudal fin being arranged symmetrically above 

 and below the axis of the spinal column. 



All M. Vogt's evidence, therefore, goes to show merely that a 

 heterocercal fish is heterocercal at a given period of embryonic life ; 

 and in no way affects the truly homocercal fishes. 



In the second place, it appears to have been forgotten that, as 

 M. Vogt's own excellent observations abundantly demonstrate, 

 this heterocercal state of the tail is a comparatively late one in 

 Coregonus, and that, at first, the tail is perfectly symmetrical, i.e. 

 homocercal. 



In fact, all the evidence on fish development which we possess is 

 to the effect that Homocercality is the younger, Heterocercality the 

 more advanced condition : a result which is diametrically opposed 

 to that which has so long passed current, but which is in perfect 

 accordance with the ordinary laws of development ; the asymmetrical 

 being, as a rule, subsequent in the order of development to the 

 symmetrical. 



The speaker then concluded by observing that a careful con- 

 sideration of the facts of Palseontology seemed to lead to these 

 results : 



I. That there is no real parallel between the successive forms 

 assumed in the development of the life of the individual at 



1 Von Bar had already pointed out this circumstance in Cyprimis, and the relation of the 

 fcetal tail to the permanent condition in cartilaginous fishes.— See his " Entwickehings- 

 geschichte der Fische,'' p. 36. 



