2"]?- GEOLOGICAL BIOLOGY. 



Characters Early and Rapidly Evolved were Chiefly Intrinsic 

 Characters. — It will be observed that almost all of the charac- 

 ters, which have thus far been considered in tracing the dif- 

 ferences distinguishing different classes, orders, families, and 

 to some extent genera, are intrinsic and not extrinsic char- 

 acters. 



Application of the Terms Intrinsic and Extrinsic to the Elabo- 

 ration of Machinery. — To illustrate the fundamental nature of 

 this distinction we may call attention to a purely mechanical 

 contrivance, the steam-engine, and the machinery run by it. 

 The force here concerned is heat, which is transformed from 

 burning wood into expansion of water into steam. The 

 simple process is the transfer of elevation of temperature into 

 enlargement of tJie space occupied by the steam. This expan- 

 sion is in every direction. The engine is a device for concen- 

 trating the direction of expansion in one line, i.e., that of the 

 axis of the piston-rod. So long as no greater elaboration of 

 the mechanism is made in the engine, it is necessary to take 

 the effect of the stroke upward only ; the production of a hinge 

 in the rod, and an attachment of the rod to a lever, make 

 the walking-beam engine, which could, at the other end, 

 work a pump; but the differentiation, which turned the link 

 into a crank, causing continuous revolution of the wheel, was 

 an intrinsic elaboration of machinery, involving a coadaptation 

 of all the parts of the machine. Improvement in the way 

 of elongation of the lever, or change of the relative size of the 

 parts, in the modification of the wheel, in the shape and rela- 

 tive size of its parts, was purely extrinsic. Again, for the 

 transfer of motion a belt and flat wheel was modified into a 

 wheel with cogs, or the reverse — I do not know which ; this is 

 expressive of intrinsic elaboration of the device, while the 

 increase of cogs in number, or size, or shape, or change of 

 relative motion by different number of cogs on the two 

 approximating wheels, is of the nature of extrinsic modifica- 

 tion. 



Summary and Conclusion. — From this illustration it becomes 

 evident why it is rational to expect a different rate in the 

 process of organic evolution from within, or intrinsic evolution, 

 from the rate of the evolution from without, or extrinsic evolu- 



