1884.] 145 [Hyatt. 



mechanical evolution, and we must also recognize the influence of 

 effort as an essential cause of modification, as suggested in Cope's 

 theory. 



Effort, either as a purely mechanical reaction in response to irri- 

 tation or excitation, or in less primitive shape, is probably one 

 of the causes of all structural modifications, the more remote 

 or ultimate cause being the direct action of the environment 

 producing the primary irritation or excitation. The older nat- 

 uralists respect the Lamarckian theory of effort when applied 

 to man, and designate all attempts to apply it to the lowej- ani- 

 mals as speculative, whereas it is not more so in their case 

 than in the history of progress in man himself. Intelligent effort 

 has no definite boundary by which it can be separated from auto- 

 matic effort, or the exercise of any of the organs or functional 

 powers on the part of man, or the lower animals, and they all 

 have their birth in purely mechanical reactions which have been 

 demonstrated by many authors, notably Semper, Dohrn, Cope and 

 Ryder. 



There is a gradation in the stages of development of the ecto- 

 derm, endoderm and mesenchym in the sponges which shows them 

 to have retained the ancestral protozoonal characteristics in some 

 cell-zoons more than in others. Thus the ectodermic cells in all 

 the Porifera become permanently transformed into flat epithelial 

 cells losing their feeding organs, the collars and flagella, whereas the 

 cells of the endoderm in some forms, such as the Ascones probably, 

 never lose these organs at all, and in others lose them onty tran- 

 siently at certain stages, or only locally on the walls of the 

 archenteron in the intervals between the diverticula or ampullae. 



In the mesenchyme the cells have been subjected to fewer changes 

 and they preserve their ancient amoeboid al forms comparatively 

 unaltered. The great change in the evolution of the group prob- 

 ably took place after the transfer of the principal seat of assimilation 

 from the endoderm to the mesenchyme. This transfer occurred 

 in the genesis of some Sycones, and other higher forms. Then the. 

 mesenchyme probably began to become thicker and to show more 

 definite tissues, as in the dermal layers in many forms, such 

 as Spongilla and Spongia, Apysilla, etc., but whether this differen- 

 tiation ever goes to the extent of producing specialized contractile 

 muscle cells having a fixed form is doubtful. The localization of 



PROCEEDINGS B. S. N. H. VOL. XXIII. 10 MAY, 18S5. 



