Willis .— The Origin of the Tristichaceae and Podostemaceae. 305 
of living in water for the secondary shoots which we have pictured to our¬ 
selves, are not hereditary. It is conceivable that something like memory 
may come in, and after they have lived like this for very long periods, may 
cause the necessary mutation to appear which will render the habit a perma¬ 
nent and practically irreversible one. In cases like Littorella —it is worthy 
of note that only genera, not families, exhibit such peculiarities—the memory 
may be as yet imperfect, and if one could watch them long enough, one 
might see a separation of a land form from a water form. 
Another great difficulty which crops up upon any theory of the origin 
of the Tristichaceae and Podostemaceae, though it is no greater with this 
than with any other theory, is to explain why there are no related forms 
which might represent the ancestors, for as the progeny would not come 
into competition with the ancestors, there seems no reason why these should 
have disappeared. It is possible that the ancestor was already a single 
isolated species, and even after having given rise to the early Podostemaceae 
continued to live mainly in the water by secondary shoots, and was defeated 
by the better adapted Tristichaceae or Podostemaceae. Or again, it is 
possible that it was killed out in some change of conditions on land which 
did not affect the water plants, at least not enough to destroy them also. 
Or again, it may be that it will yet be discovered in some out-of-the-way 
district of the tropics. 
Be these difficulties as they may, however, there seems good reason 
to suppose that these peculiar orders started as land Dicotyledons, which 
lived by the side of some of the many rapids in a warm country, and experi¬ 
mented, so to speak, with secondary shoots which they sent into the water, 
till on some lucky day—or other longer period of time—a mutation appeared 
which enabled them to live their whole life in the water from the seed, and 
thus opened up to them the virgin territory of the rocks in the rivers and 
streams of the tropical and subtropical, zone. 
Once the necessary mutation to enable the primary axis to live from 
germination in the water had been performed, this mutation would survive, 
and we should have the first unquestionable Tristichaceae or Podostemaceae. 
But having got thus far, there is, as we have seen, 1 no action of natural 
selection in the evolution of the families from this common ancestor, and the 
formation of about thirty genera and 200 species. The conditions of life are 
too uniform to allow of serious action on the part of Natural Selection, and 
there is not enough competition among the individuals or with other forms 
of life. 
Nowin the origin of these families there are one or two other important 
points that come up. In the first place it is evident that the first change, 
which turned the ancestral form (whatever it was) into one of these plants, 
1 Willis : On the Lack of Adaptation in the Tristichaceae and Podostemaceae. Proc. Roy. Soc., 
B., lxxxvii, 1914, p. 532. 
X 
