( xxxvi ) 



and tliose between the various B's ])ioloy,ic.ally equal to tlie gaps between the 

 synoecic A's and B's ? Does tlie barrier which we know to exist between the 

 synoecic A' and 15', which occnr together, exist also between A^ and A-, wliich 

 are geographically separate ? Tiierc arc many scientists who say Yes, and many 

 who say No. Let ns consider first tlu; arguments advanced for the correctness 

 of an affirmative answer. 



(1) The geogra])liical representatives A' and A- are constantly different ; 

 ergo, there is a constant barrier between them, as in the case of A^ and B'. — 

 Firstly, this is begging the question. Secondly, we have seen above (p. xxxiii) 

 that corporeal differences as such do not constitute the barrier existing between 

 A' and B'. Thirdly, in a vast number of cases it is mere guess-work to 

 maintain that A' and A^ are constantly different ; all we know of them is that 

 they are different under the special conditions under which they are living, 

 similarly as seasonal varieties may be constantly different if the conditions are 

 constant. If the constancy of the special conditions falls, the constancy of the 

 corjioreal difference between A^ and A^ will certainly or perhajis — we cannot tell 

 a jjriori which is correct — also break down. 



(2) A' and A^ are geographically isolated. ' Tiiey form therefore separate 

 biological entities which do not interbreed and fuse. — We reply, firstly, that this 

 is again a restatement of the question ; secondly, that, as there are numerous 

 cases of identical individuals (as far as there is identity in individuals) being 

 geographically separated from one another, geographical isolation as such is no 

 criterion whatever ; and, thirdly, that the facts of A' and A^ being geographically 

 separate, and A' and B' living together, constitute certainly not an agreement, 

 but, on the contrary, a difference in the relation between A' and A'^, and A' and 

 B', respectively. 



Onr arguments for geographical representatives not having a. priori the 

 same biological standing as the synoecic animals which are separated by the 

 barrier before characterised (p. xxxiii), are as follows : — 



(1) The geographically separated and morphologically distinguishable 

 representatives A', A^, A^, etc., are morphologically and anatomically more 

 similar to one another than to B\ B^, B', etc. The A's are modifications 

 of one and the same type, the B's of another, the C's of a third, etc., and 

 each type as a whole stands in contradistinction to the other. This is a 

 statement of fact, not of opinion. Now, since the existing modifications are 

 the result of evolution from the ancestral types, it follows tliat phylogeuetically 

 A'. A^, A', etc., stand in closer relation towards each other than do the 

 synoecic animals A', B', C, D\ etc., the A's forming one branch, the B's 

 another, the C's a third, etc., of the phylogenetic tree. This difference in 

 the degree of blood-relationship between the geographical rejjresentatives 

 (= geographically separate components) of one ty[)e and between synoecic 

 types is very conspicuous in all classes of animals where adequate material 

 has been conscientiously studied. From the point of view of a morphologist 

 alone, all the animals which are clearly geographical representatives of one 



