THE SIGNIFICANCE OF A NATURAL SYSTEM OF CLASSIFICATION. 465 



It may be defined as a marked change of pace in phylogeny; or a rapid succession 

 of important embryonic variations due to the upsetting of a long-established con- 

 dition of organic equilibrium; or to some organic change, insignificant in itself, 

 but which at once creates new conditions for the growth of the organs so affected, 

 or of other organs. The extent of a methallosis maybe measured by the extent of 

 the variations and by the length of the period in which they occur. 



During ontogeny there are well-marked periods of accelerated development, 

 quite independent of growth or increase in size, during which there is a rapid 

 succession of profound structural changes, followed by longer periods of slow 

 development. These periods of accelerated development are well known as 

 metamorphoses, or transformations. When seen in perspective, they appear 

 as gaps separating distinct periods of life. They represent no doubt the onto- 

 genetic repetition of periods of accelerated race development, the accelerated 

 period, in the latter case, representing the real gaps between the larger subdivisions 

 of the animal kingdom. 



If the variations that have given rise to new animal forms are indiscriminate, 

 diverging in all conceivable directions from the parent stock, then the actual 

 genealogical tree that is the result of such indiscriminate variation, and as con- 

 trolled by natural selection, should show a recognizable correlation between the 

 structure of a given group of animals and its surroundings. But this is so to a 

 very limited extent only, and only in regard to minor or superficial features of 

 organs that have long existed under other conditions. 



The underlying basic structure of the organism is in no way modified by the 

 mode of life or by the surroundings, for all segmented animals, whether they live 

 in the air, in water, or on the land, agree in their mode of growth and in the 

 relative positions of the principal organs, such as the central nervous system, 

 alimentary canal, heart, eyes, olfactory organs, etc. The same thing is true of 

 the whole great class of vertebrates, where the basic structure of the jaws, gill 

 arches, the brain, and principal sense organs, is immutable, and identical for 

 every member of the class. This established structure has rigidly defined the 

 possibilities of evolution in the past and it will control the actual development of 

 the future. 



The power of articulate speech, for example, depends on the structure of the 

 lips, jaws, tongue, larynx, and the respiratory organs, and upon a definite nervous 

 association of these parts. It is a highly characteristic faculty of man, yet the 

 organic basis of the entire complex mechanism preexists, and the framework is 

 already set up in the fishes, where the rudiments of all these organs are known 

 to occur and where they have in the main, the same mode of growth, inter- 

 relations, and nerve connections they have in man. 



In the same broad sense, the basic structure of the arthropod rigidly de- 

 termines that of the fishes, and the fishes that of man. The main highways 

 of evolution are therefore mapped out by the initial structure of the most remote 

 ancestors. To that extent, evolution is direct, orthogenic, predetermined. It 



