258 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1949 
If this is accepted as a valid inference from the material studied, 
then the difference between genera, families, orders, and classes must 
lie in the quality of the unit steps involved in their evolution. This 
is not a new idea,? though we have arrived at it from a new and un- 
expected direction. 
What is meant by the statement that the quality of the steps de- 
cides the systematic category which is evolving, is easy to illustrate 
but difficult to formulate. A good example is the evolution of the 
jaws of the fishes from gill arches. This is an extraordinarily interest- 
ing case of change of function of an organ. Its real significance, 
however, is that this change provided the fishes with a decided ad- 
vantage over their environment. It opened up new and better food 
supplies and, as Sewertzoff expresses it, “increased the general life- 
intensity of these animals.”’ A biting-mouth skeleton is important 
also as an offensive and defensive weapon. It is, I think, sufficiently 
evident that this particular change of function was full of evolutionary 
potentialities. Applying Sewertzoff’s terminology, such changes may 
be called aromorphs. 
On the other hand, if one considers the extreme adaptation of a leaf 
insect (Phyllium), one finds that it has—to use Sewertzoff’s phrase- 
ology—led to a decrease in the life-intensity of the animal. No new 
food supplies have been made available by the process of adaptation, 
and its evolutionary potentialities are nil. This type of adaptation is 
typical of the lower systematic categories. 
There is another interesting feature that. emerges from the chrono- 
logical treatment of evolution. It is that there appears to be no 
correlation between fast rates of species evolution with groups having 
a rapid succession of generations. Elephants are among the most 
slowly breeding animals known and yet their rate of evolution was 
the fastest so far found. On the other hand, Drosophila, which, 
because of its rapid succession of generations, is so much used in 
experimental biology, is a genus 50 million years old. It appears 
therefore that a rapid succession of generations must not be taken as 
a, substitute for long periods of time. It is indeed surprising to find 
that the number of generations is not the only factor ruling the rate 
of change in evolution and that this change is in a vague way correlated 
with absolute time. Does this perhaps mean that external, environ- 
mental factors are influencing evolution over very long periods of 
time? I am unable to answer this question, or to offer any other 
explanation. 
I hope I have been able to show that a study of the chronology of 
evolution is well worth the effort. The suggestions made here must 
?This has been put forward in different forms, for instance, by G. G. Simpson and R. Goldschmidt. 
