CAN WE SEE EVOLUTION OCCURRING?
ing the characteristics illustrated by their representatives in the figure. A single stock, derived by fission from a single parent, has gradually diversified itself into many stocks that are hereditarily different.
What the doctrine of evolution asserts is therefore true for Difflugia. It does gradually transform and produce new races. If this is what evolution means, we have here seen evolution occurring.
A number of other lower organisms have been studied in a similar way, and with similar results. They do not remain entirely constant. Although the process is excessively slow, they gradually transform into hereditarily diverse races, in accordance with the doctrine of evolution.
To observe such changes in higher animals and plants is much more difficult. Each generation requires a longer time; in a human life few can be observed. But a greater difficulty lies in the fact that most of the higher organisms reproduce from two parents. The two parents always differ in their hereditary constitution, so that the offspring are usually a combination of two hereditarily diverse stocks. In forming that combination, each parent loses half of its genes—that is, half of the thousand different chemicals on which depend the way it develops and its later characteristics. The remaining halves from the two parents then unite to form a new combination of genes, from which the offspring develops. For every single offspring the process is repeated, but in each case it is a different set of genes that is lost from each parent, a different set that remains. Consequently through the union of the two remaining halves there is in every case a new and diverse combination of the genes produced; so that every one of the offspring of a pair of parents differs in its hereditary constitution from every other one;
[ 29 ]
