IS94-] 375 [Poulton. 
come to possess but wliich were not potentially present at the 
beginning of its separate life. 
The first factor, therefore, is made up by clianges that are 
Avrouglit in this way. The second factor is heredity, by which it 
is supposed that these changes are transmitted ; and it is certainly 
true that if such transmission is possible, some amount of evolu- 
tion must result. You will all be prepared to admit that if these 
two factors represent facts, their co-operation must produce some 
amount of evolution. 
It is important to remember, however, that both factors are not 
undisputed, as are the three factors of Darwinian evolution. 
Although we all admit the existence of acquired characters as 
the effect of external causes upon the individual during its life, 
yet biologists are by no means agreed that these effects are 
hereditary, and, if not, the acquired character ends with the 
individual in which it arose, and, not being handed on, can never 
become a character of vhe species. It is impossible for those wh 
hold the Lamarckian or Spencerian view to escape from this. If 
it is true that such characters are transmitted, then the foundation 
of the theory is secure ; but the transmission of acquired characters 
is by no means proved. Herbert Spencer has preferred to occupy 
himself in rearing a magnificent edifice upon this foundation, 
rather than employ his acute intellect in testing its firmness and 
security in every possible way. 
So far as observation goes, all those characters which are 
beheved by many to owe their origin to the Lamarckian principle, 
are present in the individual before the beginning of its active 
life, before the operation of those causes which were believed 
originally to account for the characters. According to the 
Lamarckian theory such characters have already become heredi- 
tary ; and therefore it is of essential importance to the Lamarckian 
to prove that acquired modifications can be and are transmitted. 
Only in this Avay can he give good grounds for the opinion that 
such characters, when they occur ready-made in the individual, 
are to be explained by the action of external causes during the 
lives of ancestors. 
These are the two main theories of evolution. There are 
several othei'S, upon which I will dwell only for a moment because 
these two alone command any very large amount of attention at 
the present time. 
