220 PROCEEDINGS OF THE ACADEMY OF [April, 



The phylogenist has then to use what are generally called 

 " organs " as the basis of his comparisons. And he has to deter- 

 mine by comparison which of the organs are the most conservative, 

 and to grant them morphological value in direct proportion to their 

 degree of conservatism. This nuist be done by a synthetic group- 

 ing of the facts, without any regard as to which organs are most 

 convenient of study. He must also constantly bear in mind that 

 an organ eminently conservative in one group may be variable in 

 another. Degree of conservatism can be determined only by very 

 broad and extended comparisons, and is to be gauged by the 

 measure of change accompanying change of habit. 



The great difficulty in putting this generally accepted princi- 

 ple in practice is the difficulty in determining whether the organs 

 compared are homologous. Thus nephridia are organs of a 

 certain fimction, but they are not of the same kind, not homolo- 

 gous, in all animal groups. Therefore they are not very con- 

 servative organs, even though they are present in many groups 

 of animals. This is not, however, the place to discuss the organs in 

 detail with regard to their relative morphological values : that would 

 necessitate the reviewal of an enormous mass of facts. But it may 

 be allowable to point out briefly those organs which seem of most 

 importance in phylogeny. Omitting the intestinal organ, which 

 is very readily modified by change of mode of life (as present in 

 the female but absent in the male of Rotatoria), we find two 

 organs of general occurrence in the INIetazoa, the reproductive and 

 the nervous systems. These are phyletically not as old as the body 

 layers (ectoderm and entoderm), but both are found fi'om the 

 Cuidaria upward, and with fewer modifications than the body 

 layers. Even extreme endoparasitism, the severest of all condi- 

 tions, does not lead to the complete suppression of these organs. It 

 cannot be stated that either of them is strictly homologous 

 throughout in all Metazoa, for there is good evidence of substitu- 

 tion here, though to much less extent perhaps than in other organs. 

 On the Avhole the nervous system, its central ganglionic portion, 

 remains very conservative ; and so does the central, gonadal por- 

 tiou of the reproductive system. These two are the most conserva- 

 tive of all organs, and their structure should receive first considera- 

 tion in phyletic cla.ssification. The muscular system is as old as the 

 nervous system, muscle and nerve are synchronous, but it is well 



