366 EwART — Variation : Germinal and Environmental. 



a portion and then the ivhole of the hybrid larva; from the Spha3rechinus to its own type. 

 In other tvords, the characteristics of the hybrid offspring depend directly on the relative 

 degrees of maturity of the sexual products.''^* 



(5). Why members of a family differ. 



To return to the question, " Why is there sometimes so much difference between 

 the individual members of a family ? " it may be replied, that the individuals differ 

 because the cells from which they are respectively developed differ : the potential 

 difference of the cells being greater than the actual difference of the individuals 

 derived from them. Hence, an attempt to account for the difference between mem- 

 bers of the same family does not consist in studying selection or the changes that 

 accompany conjugation. It resolves itself into an attempt to explain in what respects 

 the cells from which the offspring are separately and independently developed differ 

 from each other. It is conceivable that the individual germ-cells entering into the 

 formation of any given family may differ both morphologically and physiologically. 

 Each germ-cell, up to a certain point, may be said to be comparable to a sim^Dle 

 protozoon. Each protozoon, however simple, and though only capable of repro- 

 ducing itself by fission, has a life-history. The life-history begins at the moment 

 of separation and ends when the process of fusion or conjugation sets in. This 

 may be very short or relatively long ; but, however short, there will be time for 

 complex metabolic changes, for minute fluctuations in its vital units. The jorotozoon 

 may be well or ill-nourished ; it may have matured {i.e.., become capable of dividing) 

 rapidly or slowly ; it may divide prematurely, or the process of fission may be 

 delayed, and, when fission does occur, there may be unequal division of the 

 nucleus. 



In the case of the female germ-cell three stages may be recognised in the 

 life-history — (1) the stage up to the discharge of the first polar body ; (2) the 

 stage characterised by the extrusion of the second polar body ; and (3) the stage 

 between the reducing division and the union of the nuclei of the ovum and the 

 spermatozoon — i.e. up to the moment of conjugation. During the first stage the 

 ovum, like a protozoon, may be ill- or well-nourished, the growth may be fast or 

 slow throughout, or, by sudden changes in temperature, &c., rapid at one time, 

 retarded at another, or slow at first and so hurried at the end that ovulation takes 

 places prematurely. Moreover, if only one or two ova are ripening in, say, the 

 right ovary, and quite a number in the left, the right ones may have the advan- 

 tage and some of those in the left may be inadequately nourished, just as ova, 



*• Proceed. Roy. Soc, vol. Ixiii., May, 1898. 



