NATURAL SCIENCES OF PHILADELPHIA. 341 



Firstly. " A low grade in the scale of organic life, implying great capacity 

 for variation. 



Secondly. "A long period of time during which these exceptional forma 

 could have been produced and perpetuated." 



The first of these conditions will be readily admitted ; the second, however, 

 which touches more directly on my hypothesis, requires some notice. Sev- 

 eral considerations might be urged in proof, of which the most important 

 seem to me to be the following : 



1. " The absence of these comprehensive forms in the sub-peat deposits, 

 and also as recent species elsewhere." 



2. " The general resemblance of most of the living species of the Saco 

 pond to those found in the sub-peat, and the very partial representation of 

 genera common in surface gatherings in the neighborhood and elsewhere." 

 This general resemblance must not be understood as implying that the Saco 

 species, or many of them at least, are not to be found as living forms in 

 other streams and ponds in this country, but that the proportion grouping 

 and prevailing varieties of species common to both do not correspond ; at 

 the same time, as before stated, surface genera, such as Nitzschia, Tryblionella, 

 Cymatopleura, Cocconema, Epithemia, Odonlidium, Amphipleura, &c, are either 

 absent or but feebly represented in the Saco. 



3. "The occurrence of irregular and eccentric forms having no analogues in 

 either fossil or recent deposits. Such a one is Actinella, (new genus, here- 

 after noticed,) and others of the new species, together with species not hither- 

 to found living in this country or elsewhere, I believe, as Eunotia hemicyclus 

 (Ehr.) Eunotia incisa (Gregory) and singular varieties of Himanli/ium. 



The presence of an abnormal or irregular type like Actinella punctata, a 

 form which seems to have nearly disappeared from other localities as a living 

 species, is not perhaps so much a direct evidence of a long continued chain 

 of modifying causes as it is of an intermediate, but no longer (elsewhere) ac- 

 tive assemblage of influences. 



In this connection, and in view of the apparent contradiction involved in 

 the continued operation of these influences in the present case, I now pro- 

 ceed to state more distinctly the explanatory hypothesis founded on. the prob- 

 able geological relations of the Saco deposit to the subjacent formation. 



It is that these forms represent a series of intermediate species peculiar to 

 the modern peat, and more specially to its earlier history, occurring only under 

 exceptional circumstances, their continuance as living organisms in the Saco 

 being accounted for by favorable influences derived from the thin stratum of 

 boggy soil, till and clay, which overlies the valley drift, and through which 

 well up the Saco springs, whose waters may thus in transitu acquire proper- 

 ties by virtue of which, conjointly with other exceptional local causes, species 

 and varieties, no longer existing elsewhere, may continue io flourish. 



I may be excused for citing one or two facts which lend plausibility to 

 this somewhat fanciful hypothesis, viz., the isolated nature of the deposit 

 on which the forms occur, and the comparative absence of the contained 

 species on the surface of the mud which paves the pond, as well as upon 

 the growth of grass and moss which lines its margin. Although several 

 smaller ponds lie in the immediate neighborhood, one being separated by a 

 few yards only from the Saco, in no case have I succeeded in finding in any 

 of them specimens of these new species, of Eunotia hemicyclus, E. incisa, or 

 even of E. diadema, a common Saco form. This same remark holds good 

 also with regard to several small mountain brooks which empty into the pond 



liar relations to another genus towards which some subtle, natural force, some intrinsic bias, is 

 impelling it. 



This bias, constituting the very essence of the intermediate type, it would aebm to matter but 

 little, should the forms expressing it be single or several between a group, sub-group, family or 

 genus ; or whether they represent a series more or less progressive, providing that the esstntial 

 l.mits of the natural division to which the type belongs be not overstepped. 



1863.] 



