i 
406 Professor H. H, Swinnerton— 
are frequently the subjects of the same limited range of changes. 
Owing to the varying rates of change of the elements in different 
lines of descent, the majority of varieties and species as a whole 
are quite distinguishable from one another. If, however, several 
elements happen to attain the same stages together in two or more 
_of these lines a resemblance, often quite striking, arises between even 
distantly related organisms. This phenomenon finds expression in 
the terms “ morphological equivalence ”’, ““ homceomorphy ”’, “ con- 
vergence’. If these same structures maintain the same relative 
tates of change, that is to say, if they become correlated with one 
another, then a series of homceomorphs will result. In any case 
the goals towards which the flow-lines of changes tend may be the 
Same in quite divergent lines of descent. In the later history of 
several such lines it often happens that first one and then another 
homologous structural element reaches its goal, and morphological 
resemblance becomes dominant. The only escape from this blind 
alley of homceomorphy and convergence is either by extinction or 
by reversal of the direction of change. 
Some of the phenomena dealt with above bring the paleontologist 
within hailing distance of the Mendelian experimenter; for both 
are approaching the same problem from different angles. It may 
not be out of place therefore to express some of the thoughts which 
a comparison of the results attained by both classes of workers 
Suggests. 
The accompanying table gives a summary of Mr. Carruthers’ 
discoveries concerning the distribution of the mutational stages 
in the evolution of Zaphrentis delanouei in time and space. In con- 
sidering the figures 1t must be borne in mind that the distinction 
between stages or mutations (Waagen non De Vries.) is arbitrary, 
and corresponds to no breaks in the series. 
Stratigraphical 
Jevel. Z. delanouet. | Z. parallela. | Z. constricta. | Z. disjuncta. 
Millstone Grit . — — 5 95 
Upper Limestone — 3 16 83 
Lower Limestone aah °3 69 28 
Cementstone 
group... 69 30 1 — 
TaBLe Il.—This shows the percentage number of representatives of each 
stage at each of the four main stratigraphical levels. 
From this it is evident that all the stages had representatives at 
the lower limestone level. Similarly at other levels several stages 
were present. Evidently, therefore, varieties which are con- 
temporaneous with one another, may give a true representation 
of the evolutionary stages ; and the differences between homologous 
structural units, shown in them, may mark out the lines along which 
changes in the units flow. 
The biclogist draws the varieties upon which he experiments 
