PRESIDENT'S ADDRESS. 49 



words of definition : I regard a type as an ideal representation of a 

 group of species which may embrace the whole of a comprehensive 

 genus, or possibly more ; or it may be only a subordinate division 

 of a genus. I do not use the term as interchangeable with any of the 

 terms which are used in systematic classification, such as species, 

 genus, family, &c. ; but sometimes it may be equal in scope to 

 any of them, as, for example, when only a single species of a genus 

 or of a family is known. It may, however, be properly substituted 

 for species, genus, &c., in cases where, as in the Ostreidse, for ex 

 ample, specific and generic diagnoses cannot be satisfactorily made. 

 I shall, in the following remarks, have somewhat frequent occasion 

 to refer to types, as just defined, and to their persistence through 

 the geological periods, for I shall assume identity of type to be 

 proof of lineal descent. 



The fact that genetic lines of descent among animals have come 

 down to the present time through successive geological periods be 

 ing admitted, we may next inquire as to the manner in which they 

 have been preserved, or rather how some of them may have escaped 

 destruction during the physical changes which have occurred since 

 those lines were established. I must necessarily make occasional 

 reference to marine faunas in the following remarks, but it is my 

 present purpose to discuss only those terrestrial and fresh-water 

 faunas, the remains of which are found within the present limits 01 

 North America. 



The manner in which lines of descent of the various families and 

 types of animals have been preserved through the geological ages, 

 and in which their perpetuation has been secured, has necessarily 

 been different in the case of different kinds of animals. The sea 

 having always occupied the greater part of the earth's surface, not 

 withstanding the shiftings of land and sea, which have, from time 

 to time, taken place ever since land and sea first appeared upon 

 the earth, one may readily understand how unbroken perpetuity of 

 marine life may have been secured from the earliest dawn of life to 

 the present time. It is not to be doubted that numberless lines of 



