182 PROF. JOHN CLELAND, M.D.^ ETC., ON 



I maintain that Cuvier's division into four is correct, not 

 merely on the grounds which Cuvier stated, but for other 

 •and it seems to me more important reasons as welL But 1 

 am confronted at once with the argument that Vertebrates 

 are divisible into those which have an amnion and those 

 which have none ; that the amnion is a structure of very 

 early occurrence in individual development, and that the 

 division which its presence or absence effects must therefore 

 be of primary importance. That argument is founded alto- 

 gether on a misconception. If, in the early development 

 of one embryo, a part of its blastema is differently developed 

 from Avhat it is in another, that difference will affect every 

 structure derived from the portion of blastema implicated. 

 That is so obvious that every one can understand it, and a 

 very slight study of abnormal developments is sufficient to 

 bring it home to every anatomist. But the amnion is not a 

 part of the embryo, it is not a part of the future animal, it 

 is an enx^elope round it, a mere complication of part of the 

 germinal membrane outside the embryo proper : and this 

 enormously diminishes its value for purposes of classification. 

 No one would ever think of classifying the animal kingdom 

 according to the characters of the ova from which different 

 kinds of animals spring. Such a procedure would break up 

 the Vertebrata altogether. The limited Reptiles, that is to say, 

 Reptilia as distinguished from Amphibia, would indeed form 

 in conjunction with Birds, a coherent group under such an 

 arrangement, but the group would be sundered as far from 

 the other vertebrates as from any of the invertebrata. 

 The fact is that the peculiarity of the ova of the so-called 

 Sauropsida does not in the least affect their morphological 

 constitution, and that is j ust what is true also of the presence or 

 absence of the amnion. It is notable in passing that, to what- 

 ever cause we may attribute the appearance of an amnion, it 

 must be considered one of those structures which have appeared 

 independently in two different stems, if it be true as is gener- 

 ally held that Mammals are not derived from Birds or the 

 restricted Reptilia. but from an amphibian or pre-amphibian 

 ancestry, a doctrine which 1 am not disposed to object to. I 

 merely mention the circumstance at the present moment 

 because it is one of those facts which support the doctrine of 

 determinate evolution. 



I know of no other argument worthy of serious considera- 

 tion, besides that derived from the absence of an amnion, 

 for separating the Batrachia of Cuvier from the Reptiles and 



