10 The Mechanism op Evolution in Lbptinotaesa 



of investigation, and hope for further historical information that shall in some 

 measure help to close this gap in our knowledge. Complete information as to 

 the nature of the form that seems so fundamentally a part of each phylum, 

 especially as to its method of production and change, is and must be entirely a 

 matter of experiment ; and it is not too much to hope that within no distant time 

 it may be possible with increased knowledge of organic constitutions and skill in 

 experimental operations to attack directly and change permanently the basal 

 system upon which the phylum is built. 



Fixity of phyletic type is axiomatic and has long been admitted, even when 

 accompanied by differences in theory as to origin and also in the method of 

 producing diversity within it. From the first to the last aU have been impressed 

 with the fact that in the history of any phylum there has been uniformly the 

 progress of the type as a whole from the simple conditions when it appeared first 

 in historical records to later periods or to the end of its existence, and this has 

 produced the idea of a movement in the series based upon the phyletic type, and 

 has found expression in different conceptions as to its causation. From Aristotle 

 to the present it has been difficult to escape the cul-de-sac of an evolutionary 

 principle or force which in the specific phylum determines and in a way drives 

 it through the observed series of conditions discovered in its history. The ques- 

 tion why may be asked ; it can not be answered at present in any certain terms, 

 and belief in any agency helps only individually in forming a comprehensive 

 and comforting picture of nature. Any theory of evolution to be valid and 

 comprehensive must in some manner provide in naturalistic operations for 

 these phenomena in the history of all phyla of living things. There is no evi- 

 dence of growth force any more than there is of a principle of perfectibility, 

 or evolutionary force, or bathmogenesis, or orthogenesis, in the production of 

 the results observed. 



Any naturalistic cause of the history of progressive specialization observed in 

 phyla must at the same time provide for the evident simplification of the mem- 

 bers of the series in the later stages of its history, and the return of them to 

 the more elementary states found at the beginning of the history of the phylum. 

 It may be interesting to compare these to the juvenile, adult, and senescent con- 

 ditions in the span of a single life, and while there may be similar processes 

 productive of the two, as far as present information is available, there is no 

 evidence that it is so, no matter how earnestly one may believe it. 



Thus at the base of our conceptions of evolution in nature there lie universal 

 conditions, beyond doubt entirely naturalistic as to production, but at present 

 known to us only from the broad outlines of their results through past history 

 in different phyla and throughout organized nature as a whole. Most probably 

 the phyla are but the expression of these same underlying causes, and could we 

 arrive at an understanding of the nature of a phyletic form and the operations 

 whereby its composition could be permanently changed through experiment, 

 there would no doubt thus be derived the basis for understanding the funda- 

 mental nature of these more proximate questions of the apparent movement and 

 the history of any phylum, and in the rise, culmination, and decline in the 

 complexity of the members of groups as the special series passes through the 

 history of the planet. 



The problems of phyla and of larger groups within the phylum are a matter 

 of phylogenetic speculation; and evolution theories have, from Lamarck to the 



