Mutations and Evolution. 
225 
sporophyte has developed far enough to make possible the production 
of spores in sufficiently large numbers to ensure the passage through 
the difficult stage in the life-cycle when water is a necessity for the 
accomplishment of fertilization. Only in vascular plants above the 
Cycads has complete adaptation to terrestrial conditions been 
accomplished, by the adoption of siphonogamy and the loss of 
swimming sperms. 1 Nevertheless, the higher siphonogamous plants 
do not, except in rare instances, show recapitulation stages of their 
gametophytes. In other words, the gametophyte development is 
usually direct. This can hardly be because the gametophyte in its 
later evolution forms a reduction series, because among animals the 
most remarkable and convincing instances of recapitulation occur 
where degeneration has taken place as a result of parasitism or the 
adoption of sedentary habits. Typical recapitulation phenomena 
do nevertheless appear in plant gametophytes. For instance the 
archegonium, which is so characteristic an organ in Bryophytes, 
and has given an aggregate name to the three great groups in which 
it occurs, persists in a progressively reduced form throughout the 
Gymnosperms until we reach the higher Gnetales, although in certain 
Araucarians (Eames, 1913) it has become a positive hindrance to 
fertilization. In this case the archegonium neck cells have developed 
into a thick-walled structure which the pollen tube cannot penetrate. 
But the jacket-cells adjacent to the neck are actually eliminated, 
thus making it easier for the pollen tube to reach the central cell. 
No more striking case could be cited of the continued production of 
an organ which not only has lost its function but which is a positive 
hindrance to the functioning pollen tube, though its retention has 
entailed other structural changes in the jacket-cells to facilitate 
fertilization. 
Thus throughout the Gymnosperms, the gametophyte is being 
reduced, and recapitulation phenomena which suggest an ebbing 
tide occur in its terminal stages. The archegonium itself shows a 
gradual series of reduction stages until it is finally eliminated. In 
the lower Gymnosperms it has already lost the neck canal cells 
found in the archegonium of Mosses and Ferns, and among the 
Conifers the ventral canal cell is gradually eliminated. In the 
Abietineae (see Coulter and Chamberlain, 1910) a ventral canal cell 
is cut off, in the Araucarians the nuclear division takes place but 
no cell wall is formed ; or in some species of Pinus a wall may be 
1 It is well known that similar types of adaptation from aquatic to 
terrestrial conditions have occurred in the Fungi, involving the loss of free 
swimming gametes and the development of some form of siphonogamy. 
