Our present actions affect the lives of future persons. We may affect their quality of life, as when a pregnant woman's smoking harms her baby. But we may also affect their very identity, as when a woman decides to have a child now rather than waiting a few more years. Such considerations give rise to the following case (Parfit, p.358):
The 14-Year Old Girl. This girl chooses to have a child. Because she is so young, she gives her child a bad start in life. Though this will have bad effects throughout this child's life, his life will, predictably, be worth living. If this girl had waited for several years, she would have had a different child, to whom she would have given a better start in life.
Although the girl's choice is obviously worse, and for humanistic reasons, nevertheless there is no person who is harmed by her choice. The child that is born receives a life worth living, and the alternative for him is to never have existed at all, so he clearly has no grounds for complaint. But it would have been better for the girl to have waited, and thus had a different child, one that would have a better life.
This Non-Identity Problem has much broader implications. Suppose we have a choice between environmental conservation vs degradation. Whichever choice we make will influence not just the quality of life of future persons, but also their identity. Suppose that after a few centuries, the two scenarios have no individuals in common. (This is plausible: If not for Hitler and WWII, I may never have existed. Even small changes to history would soon compound to create a significantly different future.) Then the individuals in the degraded scenario have no basis for complaint.
Recall: if we had chosen the conservationist route, they would never have existed! Since their lives are worth living, they must be pleased - at least for their own sake - that we were environmentally irresponsible. They have inherited a polluted world, but since the alternative was for someone else to inherit a cleaner world, the polluted option is actually better for them (or at least no worse).
Still, the path of environmental degradation is clearly worse, from a humanistic moral perspective. It may not be worse for any individual, but it is worse for humanity. People (in the general sense) could have had a higher quality of life, even if no particular person could have. We should not only be concerned about benefits and harms to particular people (i.e. individuals with a set identity), we should also care to bring about the existence of individuals with a higher quality of life than possible alternative individuals would have. We should care about family planning, population issues, environmental conservation, and so forth -- even if bad decisions here would harm no individual.
To ground these judgments, Parfit (p.360) suggests The Same Number Quality Claim:
Q: If in either of two possible outcomes the same number of people would ever live, it would be worse if those who live are worse off, or have a lower quality of life, than those who would have lived.
Principle Q entails the badness of harms, so once we accept it there is no need to take the latter as fundamental. It is a general theme of Parfit's work that the separateness of persons is not of great moral significance. The present point supports this. We can hold that it is bad to harm me because in doing so you reduce the quality of life of people generally (since I happen to be one). The fact that it is a particular person (me!) that is harmed, is of no significance here. It would be just as bad if my reduced quality of life were brought about as an alternative to someone else existing in my place with a higher quality of life. The badness is just the same, though there is no harm done to me in this latter scenario. (I am as well-off as I can be. It is someone else who could have been in better circumstances than I.) The harm plays no essential role in the bad-making, then. What really matters is more impersonal than we would initially have believed.
See also: The Future of Consequentialism, and The Population Paradox.