OCR Text |
Show 286 DR. II. GADOW ON EVOLUTION [Mar. 20, " lumpers," who, rightly allowing more amplitude of variation in their conception of a species, commit the following error. They think that intergrading of two species is the same as continuity from one extreme to the other. For instance, if the overlap of all the available characters should occur in one and the same specimen, then presumably the two supposed species would be the same, but not-and this is the usual procedure-if the overlap of the characters occurs only in a whole number of specimens taken together. The following diagram illustrates an important point. Let a, b, c, d be 4 different characters, each of which can vary from, let us say, small to large ; and let us assume that character a (for example the scutellation of the forearm) is the quickest, the most susceptible, to change. Let species A change towards B, and let B change towards A, by gradually assuming the respective characters. Then it will be found that the two changing series will overlap completely oi' coincide in all their four characters, only when all these characters have arrived at a medium condition, and again when they all have arrived at the other extreme end. The diagram shows moreover that, although the results are the same, at the terminus and in the middle, the A and B series of evolution are different at every stage. SMALL. Species A =a bed changing towards B-*- a a b a MEDIUM. d c d bed abed a b c a b a dab c=a d d a d a b bed d d a dab dab c=a b LARGE. d c d bed abed. c d A has changed into a form in which all the characters are large; A resembling B. a b c d= Species B changing into, or towards A. a b c -*-«• b c c a b c b c c Diagram illustrating the overlapping of characters. Species B resembles A, but is not genetically the same, since the combinations dab small, or a b c medium and d small, &c. occur nowhere in the series which represents the changes from A towards B. In this paper I have employed a great number of specific &c. names, often using trinomials, in fact as many as the greatest of "splitters"; but this has been done for the sake of convenience, for shortness of expression, and having done m y best to diagnose the groups, |