112 NATURAL SCIENCE [August 
(3) An accurate knowledge of the conditions under which 
the individual developmental stages occur, or the 
transformation of one form into another obtains 
This point has received as yet scarcely any attention in algae or 
other organisms, eg., fungi, and such knowledge has never been 
recognised as necessary, since it has not hitherto been believed that 
it was possible to determine these conditions. In works which deal 
with polymorphism, from Kiitzing to Borzi, great significance has, it 
is true, been attributed to external conditions in the transformation 
of forms, but only in quite a general, undefined and vague way. 
Never in any case has a given developmental form been clearly 
recognised and demonstrated as the necessary consequence of definite 
external conditions; such forms have mostly been observed merely 
by chance. In his work on Hremosphaera, Chodat describes, besides 
the well-known typical cells, certain dwarf forms with somewhat 
different structure, he describes Palmella-Glococystis-stages, he also 
brings a Chlamydomonas-form into connexion with these; all of 
which are developmental forms or (it may be) independent species 
found accidentally in the same culture of Hremosphaera. We never 
get a hint of an explanation how such various states of the same 
alga can appear in the same culture. 
But since my observations have shown that external conditions 
actually decide the appearance of the reproductive stages of many 
algae, it has become necessary in all similar work to attempt at 
least to discover the appropriate conditions of the appearance of 
each developmental form. 
The next goal to be attained is such an exact knowledge of the 
conditions that we can elicit a given developmental form at will. 
Such investigations as these naturally demand much time and 
trouble; and even so in the case of many organisms they do not 
lead to the desired result. Thus, in certain species, in spite of the 
firm conviction that external conditions must be of great significance, 
these conditions are not yet sufficiently clearly understood, as for 
instance in the case of Ulothrix zonata, Hormidium nitens, ete. 
Thus there remains a gap in our knowledge, which, later on, 
with the aid of better methods, will be filled up. On the other 
hand, there is the alternative that a given developmental form is 
produced as a result of the operation of inner causes which we are 
not able to elucidate ; in that case we shall find by experience that 
it will appear quite regularly and can always be observed at the 
appropriate stage in the life-history of the species. But in all the 
lower algae—and these are the forms I am specially considering 
here—my whole experience leads necessarily to the conviction that 
external conditions determine the appearance of each developmental 
