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, e.g., 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 Klitzing 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 Eremosphaera, Chodat describes, besides 

 the well-known typical cells, certain dwarf forms with somewhat 

 different structure, he describes Pahnclla-Gloeocystis-st&ges, he also 

 brings a Chlamydomonas-iovm into connexion with these ; all of 

 which are developmental forms or (it may be) independent species 

 found accidentally in the same culture of Eremosphaera. "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, ITormidium nitens, etc. 

 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 



