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Preface, 4

taxonomist has been revealed even more dramatically by the numerous studies of wild species under controlled garden conditions, beginning with the work of Turesson which established the ectype concept. Most species are now known to represent complex systems of genetic variation, both within and between populations. This variation is an orderly pattern, reflecting the needs of the habitats which the species occupies. Differences between species are of an equally complex nature. Some of the most significant of these cannot be recognized at all by looking at the species concerned, however minutely. They are revealed only when the chromosomes of two species are placed together in the cells of a hybrid, and there show that they cannot work together to form normal adult structures or viable gametes. Finally, garden studies within species have shown that such characteristics as the ability or inability of a plant to produce vigorous offspring through self fertilization may characteristics of reproductive biology, which by themselves have little to do with outward appearance, are nevertheless of great importance in determining evolutionary relationships. We know now that until we have determined whether a race is usually self fertilized, or whether it requires cross fertilization to produce vigorous offspring, we cannot fully understand its relationships. Moreover, in certain groups of plants the relationship between populations is dominated entirely by the fact that they have given up sexual reproduction partly or entirely and have substituted for it parthenogenesis or some other type of asexual reproduction.

When we consider the meaning of all of these discoveries to the daily tasks as well as the outlook and purpose of systematic biology, we are driven to this conclusion. If the primary