POLYMORPHISM AND LIFE-CYCLES 175 



tation to this condition, the female gametocyte ceasing to divide 

 and becoming a single macrogamete, while the male gametocyte 

 produced a swarm of minute, motile microgametes. Only in a 

 few Coccidia, exemplified by the genus Addea (Fig. 154), did the 

 gametocytes acquire the habit of association before forming gametes, 

 a habit which led in this case to a reduction of the number of micro- 

 gametes produced to four, of which one fertilizes the macrogamete, 

 while the other three perish. It is clear that the formation of 

 microgametes in close proximity to the macrogamete increases 

 vastly the chance of the gametes finding each other, and renders 

 unnecessary the production of a swarm of microgametes. 



In the gregarines, on the other hand, the gametocytes acquired 

 the habit of associating and forming their gametes in a common 

 cyst. Under these circumstances it becomes a certainty that a 

 gamete of either sex will find a partner if the gametes of each sex 

 are in equal numbers. Consequently there is seen in gregarines a 

 progressive tendency, illustrated by the examples cited above, to 

 disappearance of those characters of the gametes which are an 

 adaptation to the necessity of the sexes coming together, culminating 

 in production of gametes of opposite sexes which are perfectly 

 similar. On this view the isogamy seen in many gregarines is a 

 secondary condition brought about by the gradual obliteration of 

 adaptive differences between the gametes of opposite sexes, under 

 circumstances which render such differences unnecessary. 



The comparison of the gamete-formation in different species of gregarines 

 furnishes an instance of a progressive levelling-down of structural differentia- 

 tion of gametes, under conditions in which no such differentiation is required, 

 until an anisogamy undoubtedly primitive has been reduced secondarily to a 

 perfect isogamy. This has led to the view expressed in many quarters, that 

 anisogamy is in all cases a primitive, isogamy a secondary, condition. The 

 case of the gregarines is by no means adequate, however, to support so 

 sweeping a generalization ; the only conclusion that can be drawn from it is 

 that adaptive differences tend to disappear when the conditions to which they 

 are an adaptation no longer exist ; and the very fact that the obvious structural 

 differentiation between the gametes vanishes in such a case is of itself a proof 

 that such differentiation is not the expression of intrinsic constitutional 

 differences between the gametes, for such differences could not be annihilated 

 merely by changed conditions of environment. 



There can be no doubt that anisogamy in the form of visible structural 

 differences between the gametes of opposite sexes must have been acquired 

 very early by gametes as an adaptation to their functions. On the other 

 hand, it is highly improbable, to say the least, that the earliest gametes, 

 when the sexual process was first invented, so to speak, were structurally 

 differentiated. It must, of course, be postulated that the gametes possess 

 such intrinsic constitutional differences as would account for their behaviour 

 that is to say, their mutual attraction and union ; and in this sense anisogamy 

 may be considered as a universal and primitive phenomenon. But the number 

 of cases in which gametes are perfectly isogamous, as regards visible struc- 

 tural or other differences, is a sufficient proof that purely constitutional 

 anisogamy does not necessarily express itself in perceptible differentiation 

 of the gametes. 



