184 GENERAL EVOLUTION. 



color exist on the one side, the same will be found on the other 

 side ; even of single marked spicule of bone the same may be said, 

 so that a description of one side will answer for both, minute dif- 

 ferences being noted as they occur." 



b. Anteroposterior symmetry. 



That this is an absolute law of creation will be less readily ad- 

 mitted than in the case of double bilateral symmetry, since the ex- 

 ceptions apjoear to be so universal. Nevertheless, I believe it to be 

 as much a part of the law of repetitive nutrition as the other. 

 The anteroposterior homologies even of the human skeleton have 

 been largely demonstrated, bnt, as usual, we must appeal to the 

 lower forms for a clear view of it. In the rudimental skeletal axis 

 we find such symmetry almost perfect in the Amphioxiis, but in no 

 other vertebrate. In limbs we have it clearly indicated in the 

 Keptilian order Iclithyopterygia, and in the Piscine order Dipnoi, 

 where the anterior and posterior are scarcely or not at all distin- 

 guishable. In the scapular and pelvic arches we find it also ap- 

 proximated in the first-named orders. 



In the nervous system it also exists approximately in the Am- 

 pliioxus. It is not seen in any vertebrate, and in but few other 

 animals, in the digestive system, but it appears to exist in some 

 lower Articulata in both the resj)iratory and circulatory systems. 



c. As illustrations of exact repetition involving large portions 

 of the organism the higher Polyps may be cited, which differ from 

 the lower chiefly by the addition of similar septa and similar ten- 

 tacles. Examples of repetition of nearly the whole organism may 

 be found in many Entozoa, as Tmnia, where the cephalic segment 

 only differs from the others, the remainder or proglottides being 

 alike. The most entire repetition of structure is seen in Vibrio, 

 where the segments are all alike, there being none representing a 

 head. 



d. As an example in special details of structure, the pelvic arch 

 of Ichthyosaurus, when first created, was a repetition of the scapu- 

 lar, and the hind limb, of the fore limb. The segments of the 

 limbs of the Dipnoi are mere repetitions, the later created of the 

 earlier. The special parts of the pes and mantis of Ichthyosaurus 

 are simply repetitive efforts of growth-force joined with a dimin- 

 ishing amount. The addition of a digit, often distinguishing one 

 genus of Salamanders or Saurians from another, is evidence of a 

 similar repetitive effort. The low mammal Ornithorhynchus pos- 

 sesses but a single tooth in each jaw ; the simple teeth of armadil- 



