266 GEOLOGICAL BIOLOGY. 



lania flavesccns^ obtained from the seas about Australia; we 

 examine its shell ; we find that it is a bivalve, equilateral 

 shell, the two valves articulating, and the one larger than the 

 other, and exhibiting a perforation through the beak for the 

 protrusion of a stem-like peduncle for its attachment. All 

 these characters are peculiar to the class Brachiopoda. They 

 distinguish this individual organism from the organisms of 

 every other class in the Animal Kingdom (Figs. 59, 60, 61, 62). 

 Evolution of the Class Characters. — Whence does the Ma- 

 gellania derive these characters ? We at once say by descent 

 from the parent Magellania from which it sprang. How did 

 it attain the characters? By ontogenetic growth from an o.^^ 

 which expressed none of them. The law of heredity ex- 

 plains the appearance of the particular characters in this in- 

 dividual organism, and the law of ontogenetic growth ex- 

 plains the formation in the individual of these characters. 

 But how did they come to be at all ? or, to put the idea in 

 another form. Why is it not a clam-shell ? Heredity explains 

 why it is like its ancestral predecessors; but what explains 

 the fact that it is unlike organisms of all other classes ? In 

 answering this question we are led backwards, and find in 

 the Tertiary Period forms presenting the same characters; 

 and because there is thus traced a succession of forms with 

 the same characters, we assume that descent will account for 

 the succession. Still further back, in the Cretaceous, in the 

 Triassic, in the Devonian, in the Silurian, and even in the 

 lowest beds of the fossil-bearing series, the Lower Cambrian, 

 we find fossils possessing the essential class characters of our 

 living Magellania. There at the first stage of appearance of 

 Brachiopods the difference is obvious between the Orthisina 

 and the species of any other class than Brachiopods. We 

 can go no further for facts. We have to confess that we 

 have no knowledge of the origin of the class characters of 

 Brachiopods; we only know that they were evolved as far 

 back as Cambrian time, and that they have ever since been 

 transmitted by ordinary generation. 



Evolution of the Ordinal Characters. — In the same way we 

 notice on the hinge margin the production of two processes 

 each side of a triangular fissure which we call teeth and del- 



