260 LAY SERMONS, ADDRESSES, AND REVIEWS. [xil 



founds the specific character are constant in individuals of both sexes, 

 so far as observation has reached ; and that they are not due to 

 domestication or to artificially superinduced external circumstances, or 

 to any outward influence within his cognizance; that the species is 

 wild, or is such as it appears by Nature." 



If we consider, in fact, that by far the largest pro- 

 portion of recorded existing species are known only by 

 the study of their skins, or bones, or other lifeless 

 exuvia ; that we are acquainted with none, or next to 

 none, of their physiological peculiarities, beyond those 

 which can be deduced from their structure, or are open 

 to cursory observation; and that we cannot hope to 

 learn more of any of those extinct forms of life which 

 now constitute no inconsiderable proportion of the known 

 Flora and Fauna of the world : it is obvious that the 

 definitions of these species can be only of a purely 

 structural or morphological character. It is probable 

 that naturalists would have avoided much confusion of 

 ideas if they had more frequently borne the necessary 

 limitations of our knowledge in mind. But while it 

 may safely be admitted that we are acquainted with 

 only the morphological characters of the vast majority 

 of species — the functional, or physiological, peculiarities 

 of a few have been carefully investigated, and the result 

 of that study forms a large and most interesting portion 

 of the physiology of reproduction. 



The student of Nature wonders the more and is as- 

 tonished the less, the more conversant he becomes with 

 her operations ; but of all the perennial miracles she 

 offers to his inspection, perhaps the most worthy of 

 admiration is the development of a plant or of an animal 

 from its embryo. Examine the recently laid egg of 

 some common animal, such as a salamander or a newt. 

 It is a minute spheroid in which the best microscope 

 will reveal nothing but a structureless sac, enclosing a 



