68 NATURAL SELECTION FROM 



(archegonia) appear. In the first stage of its 

 development it consists of a large number of free 

 nuclei; in the second stage walls appear and a 

 tissue is formed ; and in the last stage this tissue 

 grows and finally produces the archegonia with 

 their egg. The constant tendency throughout 

 the whole group is to produce the female sex 

 organs earlier and earlier in the history of the 

 gametophyte. A series can be arranged illus- 

 trating the appearance of the sex organs at what 

 might be called the mature stage of the gameto- 

 phyte, at one extreme; then their appearance at 

 earlier and earlier stages of the tissue develop- 

 ment, imtil they appear with the first formation of 

 walls ; and finally, at the other extreme, the eggs 

 appear at the stage of free nuclei, so that no sex 

 organs are formed. This progressive slipping 

 back of the egg in the ontogeny of the gameto- 

 phyte holds no relation to any advantage that can 

 be detected. Certainly it holds no relation to 

 any advantage in fertilization, for that is a pro- 

 longed process among Gymnosperms, and the 

 pollen tube containing the sperms may live for a 

 season or two in the tissues of the ovule. Taking 

 the group as a whole, this is not a sporadic 

 change, occurring here and there; but the two 

 extremes I have given are the two extremes of 

 the Gymnosperm phylum. This kind of progres- 

 sive change is beyond the reach of experiment, 

 and its explanation is beyond the reach of imagi- 

 nation as yet. 



