5 o2 POPULAR SCIENCE MONTHLY 



with the upper jaw produced into a flat blade. In numerous unrelated 

 groups of fishes, she has produced genera with the parrot's beak 

 (Beams, Oplegnathus, Tetraodon), or with an imitation of human 

 incisors. In many different wholly unrelated groups she has developed 

 bony plates, hardly to be distinguished superficially from those of her 

 ancient ostracophores and dipnoans. Thus the earlier writers placed 

 with the ganoids such forms as Callichthys, Ostracion, Agonus, Pegasus, 

 Hippocampus, Gasterosteus, Peristedion, forms now known to have no 

 affinities with the extinct mailed fishes, and for the most part no 

 affinities with each other. 



Such adaptive characters do not suggest relationship. They are 

 mostly superficial only, and indicate not the origin or affinities of the 

 forms possessing them, but rather the habits of the species in question, 

 and the needs of their recent environment. 



In finding what an animal really is, that is, in tracing its ancestry, 

 we have in the words of Haeckel, mainly the three ancestral documents, 

 morphology, embryology and paleontology. Adaptive characters are 

 essentially external. The inside of an animal tells what it is, the out- 

 side where its ancestors have been. Perhaps a fuller understanding of 

 orthogenesis will relate its facts to those of ' analogous variation ' or 

 the c convergence of characters ' in unrelated forms. 



