THE VARIOUS ASPECTS OF EVOLUTION. 469 



In the creation of the great phyla of the animal kingdom, natural selection, 

 external environment, heredity, use and disuse, have played an insignificant, 

 subordinate part. They may check, or stimulate, or eliminate in a quantitative 

 manner, but they are not primarily creators of structure or of organization. 



The familiar "Deus ex machina" of heredity and natural selection, may be 

 summoned to account for the absence of organs that ought to be present, or to 

 account for the elimination of mechanisms that will not work, but they are power- 

 less to explain the method by which a given change in one organ or organism 

 creates a change in some other organ or organism. 



The creative power of internal environment is always present, always active, 

 always changing. But the basic chemical elements of life are the same as they 

 always have been; and the cosmic, inorganic environment has changed but little 

 since the dawn of organic evolution. It is the internal, organic environments, 

 and the social and communistic environments that have gained in power with the 

 progress of evolution, and that are the most important expressions of it. 



The foundations of organic structure are therefore laid down and locked 

 up within the narrow bounds of internal environment, forming a self contained 

 system that grows and creates from within, and which is essentially unmodified 

 by changes in the external environment, except those that are absolutely pro- 

 hibitive of all life. 



Social and communistic environment are later, secondary products, that 

 lend effectiveness to selection in proportion to their own evolution; that is, accord- 

 ing to the degree to which the life at large of one group of organisms is interwoven 

 or interlocked with that of others. 



Creative evolution is the progressive interlocking of one activity with another, 

 to a common end, each moulding the other, and all moved by the mainspring of 

 a common external medium. It is expressed in a perpetual flow of newly created 

 organisms, and the form and action of each one, and of its constituent parts, can 

 alone indicate the nature of the forces that have created them. 



Hence comparative morphology and phylogeny must always constitute the 

 fountain head whence comes our knowledge of creative evolution. Such prob- 

 lems as the phylogeny of vertebrates are therefore the most important ones the 

 biologist has to deal with, for on their solution depends our conception of the way 

 in which evolution actually has taken place. 



Comparative morphology has no value except in so far as it points out the 

 historic sequence of organic forms and functions, and reveals to us the trend of 

 evolution and the causes that direct and control it. In that morphology stands 

 supreme, for the thin red trail that marks the orbit of evolution is the only index 

 we have of life as it was and shall be. If our reconstruction of phylogenetic 

 lines is hopelessly wrong, then indeed will ontogeny be a false guide, and we shall 

 be left the hopeless task of reconstructing the life history of eons from the contents 

 of a test tube, or the products of the breeding pen; or tricked into the hope of 



