26 BIOLOGICAL SOCIETY OF WASHINGTON. 



for an indefinite period and eventually declines, and either per- 

 sists in a degenerate form or dies out altogether. A second 

 branch endowed with still higher capacities is developed from 

 the first and this repeats the process, and so on indefinitely, 

 higher and higher types being successively developed, carrying 

 up the system by this process of ascending sympodial 

 dichotomy. 



Persistence of unspecialized Types. — It often happens that the 

 highest organisms of the more ancient types become extinct 

 while the lower or less perfect ones persist and are found min- 

 gled with organisms of the higher types that are the dominant 

 forms of life at subsequent epochs. This fact has led those 

 who did not understand the law of types, as just stated, into 

 doubts relative to the fact of development, since the certainty 

 that organisms belonging to types that still exist, but of much 

 higher rank, formerly inhabited the globe gave rise to the 

 belief that there has been degeneracy instead of progress. To 

 escape this error it is necessary to understand that progress 

 takes place primarily through the development of new and 

 higher types of structure, embodying successively higher and 

 higher capacity for improvement, and that the archaic forms 

 belonging to lower types, and therefore, as it were, upon a 

 lower plane of life, unable to compete with those of higher 

 type, are repressed and only appear among the latter as hum- 

 ble, and, as regards their own ancestors, really degenerate 

 forms. 



We thus have a series of epochs in the earth's history dur- 

 ing each of which a different type has predominated, each later 

 type being higher in its capacity for improvement than its pre- 

 decessor. You are all more or less familiar with the successive 

 reigns of articulates in the Cambrian, mollusks in the Silurian, 

 fishes in the Devonian, reptiles in the Mesozoic, and mammals 



