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J. HESLOP-HARRISON
habitat b, the position is reversed. An experimental demonstration of
this fact would provide one type of explanation for the occurrence of
each ecotype in its characteristic habitat. It might be considered unsatisfactory, (a) because it does not follow that the measure selected is
that of primary significance for survival when the two are in competition
(some other factor, for example, the powers of seed dispersal, may be
more important in these conditions), and ( b ) because the experiment
does not reveal which factor or complex of factors in the habitats was
responsible for the difference in dry matter production, nor which
physiological mechanism in the plants themselves was responding
differentially in the two ecotypes. To proceed further with the problems
under (a) would demand an assessment of more estimates of performance, each posing its own set of questions under ( b ) . To pursue the
queries under (a) would require a factor by factor analysis of the habitats, and a function by function study of the plants. The difficulty under
(a) might be met in part by comparing the relative fitness of ecotypes
directly in competition in the different habitats, the performance
measure adopted being simply survival. Other things being equal, the
expectation is that the habitat “sieve” will once more sort them out
appropriately. An ecotype Derby of this kind would not, of course,
represent a recapitulation of the evolutionary processes which led to
ecotypic differentiation, since at no time in the course of evolution
would the end products of differentiation in specialized habitats come
together in direct competition in any one habitat. I n the ideal situation,
nevertheless, it might be expected to provide direct evidence to support
the original circumstantial case for the ecotypic differences being adaptive. It is this kind of evidence that is sought in reciprocal transplant
experiments, and in varied environment experiments such as those conducted by Clausen et al. (1940, 1948). Given this reassurance about the
adaptive value of the ecotypic characteristics acting in consort, questions of the ( b ) type still arise, namely: I n what way are they adaptive
physiologically speaking? If answers on this level are really required,
there is no escape from the need to trace direct causal connections between environment and plant responses. For the higher plant, habitat
factors are commonly regarded as being of three general kinds, climatic,
edaphic and biotic; yet it is an ecological platitude that survival is
determined by the simultaneous effect of all, the environment being
“holocoenotic” (Cain, 1944). What possibility is there, then, of linking
“character” or “response” with “factor”, when the ontogeny and
physiology of the plant are expressions of the reaction between the
entire genotype and the entire complex of environmental influences? In
the generality of cases the answer to this must be, very little; particularly when the habitat differences are small and multidimensional. Then
J. HESLOP-HARRISON
habitat b, the position is reversed. An experimental demonstration of
this fact would provide one type of explanation for the occurrence of
each ecotype in its characteristic habitat. It might be considered unsatisfactory, (a) because it does not follow that the measure selected is
that of primary significance for survival when the two are in competition
(some other factor, for example, the powers of seed dispersal, may be
more important in these conditions), and ( b ) because the experiment
does not reveal which factor or complex of factors in the habitats was
responsible for the difference in dry matter production, nor which
physiological mechanism in the plants themselves was responding
differentially in the two ecotypes. To proceed further with the problems
under (a) would demand an assessment of more estimates of performance, each posing its own set of questions under ( b ) . To pursue the
queries under (a) would require a factor by factor analysis of the habitats, and a function by function study of the plants. The difficulty under
(a) might be met in part by comparing the relative fitness of ecotypes
directly in competition in the different habitats, the performance
measure adopted being simply survival. Other things being equal, the
expectation is that the habitat “sieve” will once more sort them out
appropriately. An ecotype Derby of this kind would not, of course,
represent a recapitulation of the evolutionary processes which led to
ecotypic differentiation, since at no time in the course of evolution
would the end products of differentiation in specialized habitats come
together in direct competition in any one habitat. I n the ideal situation,
nevertheless, it might be expected to provide direct evidence to support
the original circumstantial case for the ecotypic differences being adaptive. It is this kind of evidence that is sought in reciprocal transplant
experiments, and in varied environment experiments such as those conducted by Clausen et al. (1940, 1948). Given this reassurance about the
adaptive value of the ecotypic characteristics acting in consort, questions of the ( b ) type still arise, namely: I n what way are they adaptive
physiologically speaking? If answers on this level are really required,
there is no escape from the need to trace direct causal connections between environment and plant responses. For the higher plant, habitat
factors are commonly regarded as being of three general kinds, climatic,
edaphic and biotic; yet it is an ecological platitude that survival is
determined by the simultaneous effect of all, the environment being
“holocoenotic” (Cain, 1944). What possibility is there, then, of linking
“character” or “response” with “factor”, when the ontogeny and
physiology of the plant are expressions of the reaction between the
entire genotype and the entire complex of environmental influences? In
the generality of cases the answer to this must be, very little; particularly when the habitat differences are small and multidimensional. Then