This expression too is to be taken as an ontological structural concept. (See Chapter 6 of this Division.) It has nothing to do with 'tribulation', 'melancholy', or the 'cares of life', though ontically one can come across these in every Dasein. These—like their opposites, 'gaiety' and 'freedom from care'—are ontically possible only because Dasein, when understood ontologically, is care. Because Being-in-the-world belongs essentially to Dasein, its Being towards the world [Sein zur Welt] is essentially concern.
From what we have been saying, it follows that Being-in is not a 'property' which Dasein sometimes has and sometimes does not have, and without which it could be just as well as it could with it. It is not the case that man 'is' and then has, by way of an extra, a relationship-of-Being towards the 'world'—a world with which he provides himself occasionally.2 Dasein is never 'proximally' an entity which is, so to speak, free from Being-in, but which sometimes has the inclination to take up a 'relationship' towards the world. Taking up relationships towards the world is possible only because Dasein, as Being-in-the-world, is as it is. This state of Being does not arise just because some other entity is present-at-hand outside of Dasein and meets up with it. Such an entity can 'meet up with' Dasein only in so far as it can, of its own accord, show itself within a world.
Nowadays there is much talk about 'man's having an environment [Umwelt]'; but this says nothing ontologically as long as this 'having' is left indefinite. In its very possibility this 'having' is founded upon the  existential state of Being-in. Because Dasein is essentially an entity with Being-in, it can explicitly discover those entities which it encounters environmentally, it can know them, it can avail itself of them, it can have the 'world'. To talk about 'having an environment' is ontically trivial, but ontologically it presents a problem. To solve it requires nothing else than defining the Being of Dasein, and doing so in a way which is ontologically adequate. Although this state of Being is one of which use has made in biology, especially since K. von Baer, one must not conclude that its philosophical use implies 'biologism'. For the environment is a structure which even biology as a positive science can never find and can never define, but must presuppose and constantly employ. Yet, even as an a priori condition for the objects which biology takes for its theme, this structure itself can be explained philosophically only if it has been conceived beforehand as a structure of Dasein. Only in terms of an orientation towards the ontological structure thus conceived can 'life' as a state of Being be defined a priori, and this must be done in a privative manner.3
2 'Der Mensch "ist" nicht und hat überdies noch ein Seinsverhältnis zur "Welt", die er sich gelegentlich zulegt.'
3 '... auf dem Wege der Privation ...' The point is that in order to understand life merely as such, we must make abstraction from the fuller life ofDasein. See H. 50 above.