GENESIS, HEREDITY, AND VARIATION. 369 



of them have proclivities towards formation of a vertebral 

 column. Leaving behind the great assemblage of Fishes with 

 its multitudinous types, each having special units of composi- 

 tion, we pass to the Amphibia, in the units of which there 

 exist certain traits superposed upon the traits they have in 

 common with those of Fishes. Through unknown links we 

 ascend to incipient Mammalian types and then to developed 

 Mammalian types, the units of which must have further 

 superposed traits. Additional traits distinguish the units of 

 each Mammalian order; and, again, those of every genus 

 included in it; while others severally characterize the units 

 of each species. Similarly with the varieties in each species, 

 and the stirps in each variety. ISTow the ability of any com- 

 ponent unit to carry within itself the traits of the sub-king- 

 dom, class, order, genus, species, variety, and at the same 

 time to bear the traits of immediate ancestors, can exist only 

 in a something having multitudinous proximate elements 

 arranged in innumerable ways. (2) Again, these units must 

 be at once in some respects fixed and in other respects plastic. 

 While their fundamental traits, expressing the structure of 

 the type, must be unchangeable, their superficial traits must 

 admit of modification without much difficulty; and the modi- 

 fied traits, expressing variations in the parents and immediate 

 ancestors, though unstable, must be considered as capable of 

 becoming stable in course of time. (3) Once more we have 

 to think of these physiological units (or constitutional units 

 as I would now re-name them) as having such natures that 

 while a minute modification, representing some small change 

 of local structure, is inoperative on the proclivities of the 

 units throughout the rest of the system, it becomes operative 

 in the units which fall into the locality where that change 

 occurs. 



But unimaginable as all this is, the facts may nevertheless 

 in some way answer to it. As before remarked, progressing 

 science reveals complexity within complexity — tissues made 

 up of cells, cells containing nuclei and cytoplasm, cytoplasm 



