II. PERMANENCE OF SPECIES. 63 



original central type, the tendency of ' like to produce 

 like ' will overpower and supersede the tendenc}'- to revert 

 to the original type ; — in this case, we might hold the 

 'transition of species ' to be a theory founded on facts. 

 At present, it is scarcely more than a plausible hypothe- 

 sis, invented to account for facts, and accounting for 

 them better than any other hypothetical suggestion 

 hitherto has done." (Phytologist, vol. ii., j)p. 225 — 228). 



A recapitulation of the points at which we have arrived 

 in the preceding sections (12 3 4 5) will show these 

 results: — 1. S^tecies are not proved to be permanently 

 and unchangeably distinct in nature. — 2. During a short 

 section of time, as one or more centuries, species do 

 appear to continue the same, and to rej)roduce con- 

 tinuously their own like ; the variations therefrom con- 

 stantly tending to return to a form which is assumed to 

 be a normal or natural type (" central type ") of the spe- 

 cies. — 3. Technical species, or those described in books 

 by technical botanists, are theoretically supposed to cor- 

 respond with the natural species. — 4. But very few of 

 such technical species are ever subjected to those tests 

 which are deemed adequate to establish natural species. 

 — 5. In practice, the technical species are instituted 

 chiefly or solely upon difi"erences of physical and external 

 characters, without knowledge whether those differences 

 are casual or constant, and accompanied or unaccompa- 

 nied by the physiological characteristics of natural sjie- 

 cies. — 6. On this account reasoning men must regard the 

 technical species described in books as conjectural rather 

 than certain ; their complete identity with natural species 

 being usually left in doubt, and in many instances dis- 

 puted. — 7. Botanists of authority, both cotemporaries 

 and successors, often ditter widely about the validitj' of 



