150 NEW YORK STATE MUSEUM 



free and predatory condition. Once the dependent habit is es- 

 tablished the capacity for reaction grows weaker; degenerative 

 adaptation creeps still further back in the life of snccessive genera- 

 tions and the degradation of the adult state becomes more pro- 

 found. 



The all pervading conditions of symbiosis and dependence in 

 living creatures are largely beyond the reach of our present in- 

 quiry. We are endeavoring to seek some clew to the origin of 

 dependent hfe from its earliest and simplest expressions. The 

 parasitic conditions of the present organic world are complicated 

 in the extreme as a result of progressive and easy adaptation; 

 often two, three and sometimes four hosts are necessary to the full 

 life course of the dependent. Usually these present extreme con- 

 ditions are expressed only by soft -bodied terrestrial organisms. 

 The evidences of dependent life presenting themselves to . the 

 paleontologist must be chiefly of marine origin and wholly adapted 

 to a single host; they must moreover be wholly simple in their 

 expression or may be easily misconceived. There are certain of 

 these simple expressions of long standing; we find them in existing 

 nature and the ancient faunas show that such associations began 

 far back in the history of life. To some of these we shall make 

 special reference. Besides these a multitude of illustrations of 

 dependent and attached forms of organisms can be drawn from 

 every hand in the ancient as well as the recent faunas. They call 

 for no special illustration but they nevertheless enforce our con- 

 sideration of the origin of this condition. 



So far as our facts go there are but few evidences of true parasitic 

 conditions in the Paleozoic faunas. The oldest and clearest is the 

 well known case of the coalition of the limpetlike snail Platyceras 

 and the crinoids. The snail settles down at an early age on the 

 dome of the crinoid, placing the aperture of the shell over the anal 

 vent of its host and remains attached for an indefinite period of 

 its subsequent life. , 



It is clear that the snail depends for its food on the waste from 

 the crinoid and the fact that it remains attached for a very con- 

 siderable period of its existence is shown by specimens of the 

 crinoid dome bearing successive scars made by the enlarging 

 growth of the mouth of the snail shell. Though this is the most 

 extreme expression of ancient parasitism known to us, it was 

 evidently of a very elastic kind and by no means affected all indi- 



