188 LECTURES ON BIOLOGY 



acquire changes in its germ-plasm, and if these changes appear 

 afterwards in the body of the descendants, it is none the less a 

 transmission of acquired characteristics. It is only essential 

 that the acquired change is permanent, and is not lost with 

 the disappearance of the cause. Hertwig justly says that 'a 

 possession is what one has acquired ; an inheritance, that which 

 passes to the descendants. How the possession was acquired 

 can in nowise affect the question whether inheritance shall be 

 called apparent or real.' Hertwig further says that the quoted 

 ' instance of the temperature-experiments with butterflies leaves 

 no doubt whatever that the acquired characters of the 

 parents were transmitted to the descendants.' The possibility of 

 a transformation of the species in the sense of Lamarck receives 

 strong support from these experiments. Selection plays, of 

 course, a certain part, because among the numerous normally 

 coloured butterflies he selected those that exhibited modifications. 



Engelmann has further shown that it is possible to alter 

 the natural colour of certain Algae (Oscillatorid) by exposing 

 them some time to coloured light. Very soon the little plant 

 assumes a complementary colour. If, for instance, we expose 

 an Alga for several weeks to green light it becomes red, and 

 retains this colour even when it is kept once more in ordinary 

 light. As during this process cell-division, i.e., reproduction, 

 continues, and as the daughter-cells similarly retain the red 

 colour, it is clear that in this case, too, the change has become 

 hereditary. 



It is known that firs and larches on mountains grow more 

 slowly and form thinner rings than those that grow on the 

 plains. If we sow the seed of Alpine forms in districts slightly 

 above the level of the sea, it will be found that they retain their 

 acquired characters, that is to say, they will differ from the 

 plant, the seed of which was ripened in the plain, by slower 

 growth and thinner rings. 



For the origination of new species by external influence 

 it is not even necessary that the newly-acquired characters 

 are really transmitted. It is sufficient for the purpose that a 

 change in the conditions of life is able to cause changes which 

 remain constant so long as the changed conditions remain. 

 If we, for instance, assume that our climate had become colder, 



