28 PATTEN. [Vou. XII. 
Causes of Variation. 
It is assumed that all the embryos were at first normal in exter- 
nal appearance, and that the condition in which they were found 
was attained by a gradual fusion and atrophy of organs, according 
to the methods described under the various headings. But we 
have always had in mind the possibility, indeed the strong proba- 
bility, that in some cases the normal form was not actually ex- 
pressed, but that the embryo from the first appeared in some one 
of the various stages of fusion and degeneration. For example, 
when a particular appendage is completely or partially invagi- 
nated, we have no means of determining whether it was first 
normally formed and was subsequently invaginated, or whether 
at the moment of its first appearance it gradually assumed the 
condition in which we find it. The one condition may be 
regarded as the resultant of the simultaneous action of normal 
and abnormal factors, the other the resultant of the abnormal 
factors acting after the normal ones have found expression. 
No number of cases, however great or however varied, can 
settle that point. But a complete series of them properly 
arranged may be taken to indicate the successive stages a 
fully developed normal appendage would assume when subse- 
quently invaginated, and the same applies to any of the other 
modifications that have been observed. 
All variations of the same class are no doubt due to the 
action of similar abnormal factors that tend to throw the 
normal mechanism out of its beaten track. But while simi- 
lar factors will tend to produce similar divergences in the 
end, the nature of the preceding forms will depend on the 
intensity of their action and on the period in the develop- 
ment at which they begin to act. The variations first to 
appear are of the greatest import, because “the ‘at | first 
slight divergence becomes continually greater till it leads 
to impossible combinations which may necessarily be fatal 
to the future organism. Moreover, an organ on its way 
toward a position of stability is more readily affected by 
external agents than one that has settled down as it were 
into that mature and stable condition characteristic of all older 
