Punnett on Mendelism and species

The wonderful Project Gutenberg has just released a fully HTMLised version of R. C. Punnett's (he of the famous "square") 1911 book Mendelism, which shows how quickly the implications of Mendelian genetics, rediscovered 11 years earlier, were worked through. It's a wonderful read, and anyone with a slight knowledge of biology and the interest to work through the examples can understand it, something one cannot say of texts on science for very much longer after this. I was particularly interested in the following passage, from page 150:

One last question with regard to evolution. How far does Mendelism help us in connection with the problem of the origin of species? Among the plants and animals with which we have dealt we have been able to show that distinct differences, often considerable, in colour, size, and structure, may be interpreted in terms of Mendelian factors. It is not unlikely that most of the various characters which the systematist uses to mark off one species from another, the so-called specific characters, are of this nature. They serve as convenient labels, but are not essential to the conception of species. A systematist who defined the wild sweet pea could hardly fail to include in his definition such characters as the procumbent habit, the tendrils, the form of the pollen, the shape of the flower, and its purple colour. Yet all these and other characters have been proved to depend upon the presence of definite factors which can be removed by appropriate crossing. By this means we can produce a small plant a few inches in height with an erect habit of growth, without tendrils, with round instead of oblong pollen, and with colourless deformed flowers quite different {151} in appearance from those of the wild form. Such a plant would breed perfectly true, and a botanist to whom it was presented, if ignorant of its origin, might easily relegate it to a different genus. Nevertheless, though so widely divergent in structure, such a plant must yet be regarded as belonging to the species Lathyrus odoratus. For it still remains fertile with the many different varieties of sweet pea. It is not visible attributes that constitute the essential difference between one species and another. The essential difference, whatever it may be, is that underlying the phenomenon of sterility. The visible attributes are those made use of by the systematist in cataloguing the different forms of animal and plant life, for he has no other choice. But it must not be forgotten that they are often misleading. [Emphasis added by me]

It's my opinion that the species problem arose around this time when people started to ask the sorts of questions Punnett raises here: what makes the species genetically? It's also worth noting that he distinguishes between the diagnostic genetic factors that are used to identify species, and those that are causally constitutive, as it were. And finally it's worth noting that this is a fully fledged "biological" species concept, 29 years before Mayr published his.

Hattip to Christopher Elliot at Foundations of Science, Sydney