NATURAL SCIEXCES OF PHILADELPHIA. 245 



Second. Nov\-, the more nearly allied genera are, the more surely will these 

 generic steps be found to fall into the direct line of the steps of the develop- 

 ment of the hio-hest, or that with the longest scale, the former being truly 

 identical with the latter in generic characters. Less allied genera will offer 

 au inexact or incomplete imitation of such identity, — some additional charac- 

 ter being present to disturb it. Such genus belongs to another series, charac- 

 terized by the disturbing feature, whose members, however, bear to each other 

 the relation claimed above for such. 



The relation of genera, which are simply steps in one and the same line of 

 development, may be called exact parallelism, while that of those where one or 

 more characters intervene in the maturity ol^ either the lower or higher genus 

 to destroy identity, maj' be called incomplete paralleUsm. 



The latter relation has been dwelt on by Yon Biir, Agassiz and other writers, 

 but none have accepted the existence of exact parallelism, or seen its important 

 relation to the origin of genera. 



Third. That the lowest or most generalized terms or genera of a number of 

 allied scries, will stand to each other in a relation of exact parallelism. That 

 is, if we trace each series of a number, up to its lowest or most generalized 

 genus, the latter together will form a series, similar in kind to each of the sub- 

 series ; i. e. each genus will be identical with the undeveloped conditions of 

 that which progresses the farthest, in respect, of course, to the characters 

 which define it as a series. 



Those characters of the skeleton which we are accustomed to call embry- 

 onic, are only so because thej^ relate to the developmental succession witnessed 

 in animals at the present time. Characters not so called now were probably 

 as much so at one period now passed. Hence embryonic characters of the 

 bony system do not, as I have often had occasion to observe, characterize the 

 types of the highest rank, but only subordinate divisions of them. Thus the 

 Elasmobranchs are probably repressed forms of groups of a really higher 

 grade than the bony fishes, or Teleostei, which may l)e known to us. In their 

 early presence in the geologic series we have evidence of the first beginning of 

 a higher type. 



In the same manner it has been discovered that the molecular constitution 

 of the elementary substances do not characterize their highest or most distinct 

 series, but rather the substances themselves within the lower group or family 

 to which they belong. The gaseous, liquid and solid molecular conditions 

 being characters distinguishing otherwise allied substances in the same way 

 morphologically (we cannot say yet developmentally), as the cartilaginous, 

 osseous and exostosed or dermosseous characters distinguish otherwise nearly 

 allied genera. 



The '' family " group embraces one or many of such series. If we trace the 

 series in several families to their simplest or most generalized terms or genera, 

 and compare them, we will not find the relation to be one of exact parallelism 

 in the series of the "order," so far as our present knowledge extends, but in a 

 developmental sense, one of divergence from the commencement. 



If we could know the simplest known terms or famil}' characters of a number 

 of groups of families, or " orders," we would probably find them to represent 

 a series of exact parallelism, though to find such simplest terms we must go 

 far into past periods, since the higher the group the more extensive the rano-e 

 of its character, and the less likely to be found unmixed with additions and 

 extensions, in modern times. 



Finally, the series of classes is in the relation of the essential characters of 

 the same, as expressed in their now extinct, most generalized and simjjle repre- 

 sentatives, also one of "exact parallelism." 



a.. Examples of exact parallelism. 

 * In generic series. 

 1. As an example we may take the genus Trachycephalus (Batrachia Anura). 



1868.] 



