The important distinction that must be made between Jean-Baptiste Lamarck's position and Charles Darwin's is that the former believed that animals acquired characteristics. In other words, organisms or animals can change while alive. Darwin, on the other hand, stressed the inheritance of characteristics, not their acquisition during the existence of animals.
Nonetheless, surely an animal has to acquire a characteristic before it can be passed onto - or be inherited by - future generations. Yes, that's true – though only over time. That is, individuals don't acquire characteristics over life-times. Though over time species may acquire characteristics. Those characteristics, though, will be too small to be noticed by one generation and will certainly not be noticeable over the lifetime of an individual animal.
Is this true of all species? What about the microscopic ones which have very short lifespans? Is is literally impossible for one such species to acquire a characteristic during its own lifetime?
The Lamarkian position is that “the constant craning of a giraffe to reach leaves high in a tree would alter its sperm or egg that its offspring would be born with longer necks” (114). This seems like a ridiculous idea – though only in retrospect! That is, only in the retrospect provided by knowledge of Darwin's theories. Nonetheless, the argument is still that repeated behaviours or habits of animals has a literal affect on sperms or eggs. Thus if the sperm or eggs are affected by this behaviour, then they will automatically produce offspring that will be different in some small or even large way.
Darwin's position, on the other hand, is that there is no direct relation between animal behaviour and changes in that animal's sperm or eggs. What actually is argued about behaviour X (say reaching the higher leaves) is that it's more likely to survive and thus pass on its genes because of behaviour X. The eggs or sperm aren't changed due to behaviour. Though the behaviour leads to a situation in which that animal, and animals like it, are more likely to survive. Thus giraffes with longer necks are more likely to survive. And, because of that, those giraffes which have longer necks are more likely to pass on the long-necked gene than those giraffes with shorter necks. Thus, over time, short-necked giraffes die out because less of them survive. And the less of them that survive (due to having short necks) means that they can't pass on their genes. Short-necked genes aren't passed on; though long-necked genes are.
Thus behaviour doesn't affect genes. What does affect genes, in fact, is entirely random. Though if a random change in the structure of genes produces giraffes with long necks, and long necks are more likely to secure survival, then the genes for long necks are more likely to be passed on simply because giraffes with longer necks are more likely to survive than giraffes with shorter necks.