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The real units are the elementary species; their limits often apparently overlap and can only in rare cases be determined on the sole ground of field observations. Pedigree-culture is the method required and any form which remains constant and distinct from its allies in the garden is to be considered as an elementary species.
Our pedigree-culture was mainly an experimental demonstration of the validity of conclusions, which had previously been deduced from such observations as can be made after the accidental birth of new forms. From these facts, and even from these pedigree-experiments, it is scarcely allowable to draw conclusions as to the origin of real species.
I made a pedigree-culture of lata during eight successive generations, pollinating them in different ways, and always obtained cultures which were partly constituted of lata and partly of lamarckiana specimens. But the latas remained lata in all the various and most noticeable characters, never showing any tendency to gradually revert into the original form.
It is very interesting to note that all depends on the question whether one has the good fortune of finding a rich race or not, as this pedigree-culture shows. Afterwards everything depends on treatment and very little on selection.
Summing up our discussion of this first point we may assert that repeated selection is only selection on a small and practical scale, while a single choice would require numbers of individuals higher than are ordinarily available. A second discussion in connection with our pedigree-culture of corn is the question whether the amelioration obtained was of a durable nature, or only temporary.
Having thus indicated the general outlines of the subjects I shall deal with, something now may be said as to methods of investigation. There are two points in which scientific investigation differs from ordinary pedigree-culture in practice. First the isolation of the individuals and the study of individual inheritance, instead of averages. Next comes the task of keeping records.
According to the general rule of pedigree-culture, the seeds of each individual plant are always saved and sowed separately. This is done even with such species as the clover, which are infertile when self-pollinated, and which are incapable of artificial pollination on the required scale, since each flower produces only one seed. My clover was always left free to be pollinated by insects.
And before describing the details of my unsuccessful pedigree-culture, it may be as well to give some more instances of what occurs in nature. Stray anomalies are of course rare, but not so rare that they might not be found in large numbers when perseveringly sought for.
Seed-growers receive many complaints from their clients on this account, but they are not able to remove the difficulty. This experience is in full agreement with the experimental evidence given by the snapdragon, and it would certainly be very interesting to make a complete pedigree-culture with the radishes to test definitely their compliance with the rules observed for striped flowers.
And it is easily seen that this rule agrees with that given above, and which was followed in my pedigree-culture. Furthermore it is seen that there is a complete agreement between the law of periodicity and the responses of the deviations to nourishment and other conditions of life.
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