7 24 THE POPULAR SCIENCE MONTHLY. 



Definition of Evolution. Evolution is (I) continuous progres- 

 sive change, (II) according to certain laics, (III) and by means of 

 resident forces. It may doubtless be denned in other and perhaps 

 better terras, but this suits our purposes best. Embryonic develop- 

 ment is the type of evolution. It will be admitted that this definition 

 is completely realized in this process. The change here is certainly 

 continuously progressive ; it is according to certain well-ascertained 

 laws ; it is by forces (vital forces) resident in the egg itself. Is, then, 

 the process of change in the organic kingdom throughout geologic 

 times like this? Does it correspond to the definition given above? 

 Does it also deserve the name of evolution ? We shall see. 



I. Every individual animal body say man's has become what it 

 now is by a gradual process. Commencing as a microscopic spherule of 

 living but apparently unorganized protoplasm, it gradually added cell 

 to cell, tissue to tissue, organ to organ, and function to function ; thus 

 becoming more and more complex in the mutual action of its correlated 

 parts, as it passed successively through the stages of germ, egg, embryo, 

 and infant, to maturity. This ascending series of genetically con- 

 nected stages is called the embryonic or Ontogenic series.* 



There is another series the terms of which are coexistent, and which, 

 therefore, is not in any sense a genetic or development series, but 

 which it is important to mention, because to some degree similar to 

 and illustrative of the last. Commencing with the lowest unicelled 

 microscopic organisms, and passing up to the animal scale, as it now 

 exists, we find a series of forms similar, though not identical, with the 

 last. Here, again, we find cell added to cell, tissue to tissue, organ to 

 organ, and function to function, the animal body becoming more and 

 more complex in structure, in the mutual action of its correlated parts, 

 and the mutual action with the environment, until we reach the highest 

 complexity of structure and of internal and external relations only in 

 the highest animals. This ascending scries may be called the natural 

 history series ; or the classification or Taxonomic series.^ The terms 

 of this series are, of course, not genetically connected ; at least, not 

 directly so connected. In what way they are connected, and how the 

 series comes to be similar to the last, we shall see by-and-by. 



Finally, there is still a third series, the grandest and most funda- 

 mental of all, but only recently recognized, and therefore still imper- 

 fectly known. Commencing with the earliest organisms, the very 

 dawn of life, in the very lowest rocks, and passing onward and upward 

 through Eozoic, Palaeozoic, Mesozoic, Cenozoic, to the Psychozoic or 

 present time, we again find first the lowest forms, and then successive- 

 ly forms more and more complex in structure, in the interaction of 

 correlated parts and in interaction with the environment, until we 

 reach the most complex internal and external relations, and therefore 



* Onios-gcnnao (individual-making, or genesis of the individual), 

 f Taxis, nomos (relating to science of arrangement). 



