428 Ganong . — The Comparative Morphology of 
response to outside conditions of light, moisture, &c., in each 
generation, yet in those characters which are hereditary, 
i. e. the ground-form, relative positions of the parts, their 
order of appearance, &c., there can be no perceptible change 
in one generation. Secondly, the germination of the seeds 
at home in the desert must take place at the rainy season, 
for then only is the necessary water available. Now the 
conditions of the desert in the cloudy time of the rainy 
season are not so very different from those of our greenhouses. 
This similarity is in a manner confirmed by the fact that 
the young embryos will not grow well if exposed in our 
greenhouses to too bright light, showing that at home they 
must grow at first in shade. Thirdly, seeds of the same 
species have been grown by us under very different con- 
ditions of moisture and soil, even to such extremes as sand, 
peat, and sawdust, and despite this the seedlings vary 
extremely little from one another, except when ‘ drawn ’ 
through lack of light, or dwarfed through some kind of 
starvation. Some of these conditions must be as far removed 
from one another as desert from greenhouse. I think that so 
far at least as concerns the stages before the seedlings begin 
to depend upon new food-supply made by their own chloro- 
phyll, they are much alike under all conditions. Fourthly, 
we have raised many of these seedlings into adult stages ; 
and these, except for too great slenderness in some Opuntias 
and too little colour in some reddish kinds, are not dis- 
tinguishable from the wild forms of the same species which 
we find at home in the desert. If the adult stages thus 
correspond under long-continued differences of conditions, 
much more must the young stages under very short periods 
of difference. I believe therefore that seedlings raised in 
greenhouses with all the light that is good for them, are 
safe examples of the seedlings of those species in a wild 
state, the differences being too small to be appreciable in 
such a study as the present. Nevertheless I freely admit 
that such studies as these would be much better carried on in 
the native homes of the plants, for which a properly equipped 
